Control Systems – Sensor Control

March 29, 2011

Lighting has come a long way from 100 FC on a task plane while burning 24/7/365.  Now, we’re lucky if we can provide the client 30 FC, a task light, and some form of automatic shut off without the Power Police descending on our doorstep.  The days of supplying a light switch to grant just the user control of a space have dwindled and gone.  I’m not saying there’s anything wrong with a switch, I’m just saying that you better have some sort of automatic backup. 

With the advent (and in some cases, enforcement) of ASHRAE 90.1, very few spaces in building are mandated without a required “automatic off” function for the lights.  Whether by time-based or occupancy-sensing control, ASHRAE doesn’t care as long as the lights switch off by themselves after the set time limit.  Good news for the world’s energy store, bad news for designing engineers, only codes that are more stringent are being introduced to the masses.  And technology, it is quickly advancing past what most would consider to be the “simple” control solution. 

So where is this rambling going?  I want to achieve two separate goals through a number of posts over the next couple days.  First, I want to make you more aware of the different “tiers” of control system that is available at your fingertips.  Control solutions range from simple to complex (or perceived to be simple and perceived to be complex).  Systems have emerged to simplify user control or contractor installation (and a lot of times both), and it is important to understand these tiers so you can provide proper recommendations for specific applications.  The second goal is to better acquaint you with the palette of control tools that is available to you through the Federated line card.  Made of all flavors and sizes, the controls manufacturers were selected so that you can mix and meld to meet those simple or complex applications that are placed before you. 

Let’s start with some systems overview. 

When looking at systems logically, there are three separate tiers where the majority of systems fall.  Okay, so according to my logic there are three tiers that these systems fall into.  Now, of course there will always be exceptions, but the majority of systems can be broken down into these three types:  Sensor Control, Centralized Control, and Individual Fixture Control. 

Let’s start simple. 

And Sensor Control is really as simple as it sounds.  With this control type, you are providing local control to rooms and lighting zones.  No fancy processors, no fancy connections, just low voltage wiring and a sensing device. The easiest way to look at this solution is to look at an individual room.  If you want to turn the lights on/off based on human presence, then you install an occupancy sensor.  If you’re looking to control lights because of the amount of daylight in the space, we’re talking about a photocell.  These are individual devices that will be attached to a group of lights or all the lights in the room, and those lights will be controlled off of the sensor inputs/outputs. 

The control of this type of system is achieved through the wiring.  Sensors are either wired in line with the lights or they are wired to power packs via low voltage wirings.  With low voltage sensors, the power pack is then placed in line with the lights. 

A Sensor Controlled system is not stupid; it is just limited by the intelligence of the sensor itself.  Sensors can do many different things, including having different sensing capabilities, control multiple zones, and even dim  zones to different levels.  Control is always limited by the way the system is wired.  If a light is wired separately from a sensor, the only way to control it by the sensor is to rewire the zone or sensor.  I don’t want you to call me a liar later, so I’m going to give you a sneak peak.  I said sensors are limited by their intelligence and wiring.  Systems can be provided that allow the sensors to do both, we are just no longer talking about a sensor controlled system. 

Sensor control is really the most common form of lighting control, and its necessity is often dictated by one thing: cost.  Of all the options, this solution is by far the least expensive way of meeting all code requirements with regards to automatic shut off.  However, this system has the most amounts of limitation.  The sensors will control what they are wired to, and they will only operate to the level the sensors can operate at.  Yes, the sensors can be adjusted, but they must be adjusted individually and it is recommended those recommendations are made during installation.  Full building adjustments can only be made by reprogramming every sensor individually.  You will not make any friends with the contractor or building manager explaining that one…

So Sensor control is the simplest type of lighting control.  We’re going to break here, and come back a little later with the next tier of control, a centralized lighting system.


Electronic vs. Magnetic Dimming

March 21, 2011

A few weeks ago, we talked a little bit about Dimming and what can make your dimmers hum or flicker.  One of the underlying themes to that post was that dimming really isn’t that simple. 

Traditional dimming (Sine Wave Dimming) is a very simple concept.  In order to control the light output from a lamp, vary the voltage.  Ohm’s Law made the solution even simpler.  With Voltage being the multiple of Ampacity and Resistance, simply vary the resistance at the light to control its output.  Increase the resistance, decrease the output.  And, the converse is true as well, decrease the resistance, increase the output.  When looking at an electrical sine wave, you are either increasing or decreasing the amplitude of the wave to affect the output. 

Enter Solid State Dimming.  Using a different approach, solid state dimmers cut off a portion of each cycle of the alternating current.  The thermal inertia of the filament averages out the brightness at a lower lighting level, with no perceptible flicker.  This leads us to our main point, the two main types of solid state dimmers, Electronic and Magnetic. 

With Low Voltage lighting systems, the lamps are driven by a magnetic transformer or by an electronic transformer.  120V power is provided into the lamp, and the voltage is stepped down to drive the lamp.  With two different potential types of transformers in the lamp, it is very important to confirm the transformer type prior to selecting a dimmer, because of the different characteristics of the different dimming types. 

So let’s talk Magnetic first.  These transformers will step the 120V down to 12VAC or 24VAC, and magnetic transformers use an inductive core (steel wound with copper). 

Electrical transformers are similar, in that they will step the 120V down to 12VAC or 24VAC.  However, the electrical transformers are comprised of electrical circuitry that is capacitive by nature. 

Alright, so we have two different types of dimmers that step the voltage down to the same low voltages, but do it with different cores.  Because they have different cores, they dim the lights through a different process. 

If we go back to some of what was discussed in the previous dimming discussion, you’ll recall that electricity travels in a wave form.  When lights are at full output, the sine wave is continuously providing power to the light bulb.  When a dimmer is introduced to the circuit, the dimmer “chops” up the sine wave.  The dimmer will hold the electricity output to the circuit at 0 for a portion of the wave.  The longer the dimmer holds the wave to 0, the lower the amount of energy transferred to the circuit, the lower the light output of the lamp.  See the image below to demonstrate how the sine wave is affected by a dimmer. 

  So back to Magnetic and Electronic dimming, how do they dim the circuit differently?  Magnetic dimmers use forward phase dimming, and electronic dimmers use reverse phase dimming.  What is the difference between the two?  Forward phase dimming cuts off the front side of the wave, while reverse phase cuts off the back end of the wave. 

Forward Phase Dimming

Reverse Phase Dimming

So where does this leave us, and more particularly, where does it leave the specifying engineer or lighting designer?  We are left with two different ways to dim load types, and we are stuck with another form of coordination.  A reverse phase dimmer will not dim a forward phase lamp, and vice versa.  When dimming loads, coordinate dimmer type with lighting type to ensure a functioning lighting control system.

Daylighting 102

March 14, 2011

Daylighting 102

For the introduction to Daylighting, ways to introduce light into the space, and some of the benefits, please see Daylighting 101 (should be about 2 posts ago).  Any comments or questions, please feel free to post below.  For now, we are going to pick up where we left off with and take a look at how the systems work, energy codes and the effect on daylighting, and what makes a particular system successful. 

So we look at Daylighting, bringing daylighting into the space, we have to start talking about Daylight Harvesting.  Harvesting is defined by the use of sustainable architecture that reduces the use of artificial lighting when natural daylight is available.  When using Harvesting techniques, the daylighting sensors are set to measure in one of two ways: an open loop or a closed loop. 

Let’s start with an open looped sensor.  An open looped system is designed to measure only daylight contribution to the space.  An open loop sensor is typically placed in a position where the sensor can see only direct sunlight: on the exterior wall (facing out), on the roof (measuring daylight), or inside a skylight (placed to measure the light in the skylight). 

Alright, so going back to the closed loop system, we have a second type of system.  A closed loop system measures daylight in a space as well as the artificial light.  Closed loop systems differ from open loop, as they are typically used to measure light within a room.  Placed in a location where they will not get over exposed to sunlight or indirect light, the photocell measures all the light received on a surface. 

So now we have the options for the types of photocell that can be used to determine the daylighting in the space.  Closed loops should be utilized in spaces like offices and conference rooms where lighting levels need to be carefully monitored for important tasks.  Open loops will be utilized for spaces such as lobbies and exterior spaces.  But now that we have the proper type of sensor for our space, we need to look at the type of daylighting control these sensors will utilize. 

When controlling lights in spaces with daylight, there are three options to control the space: Switching, Bi-level switching, and dimming. 

Switching is the simplest solution, turning the lights off when there is enough daylight contributed to the space.  It is the simplest design, carries the lowest design cost, and easiest to commission.  Draw backs to the design have the most effect on the occupant of the space.  The occupant is limited to two light levels (on and off), high amounts of cost savings are hard to come by, there is little flexibility in the design, and lights switching on/off can be quite irritating.  These systems are typically used in Corridors, Atriums, and Bathrooms, spaces with non-stationary tasks and typically void of workers. 

The next level past switching is bi-level switching.  Bi-level switching still turns the lights on/off, but also offers levels of control between on/off.  These systems have more than 3 levels of lighting, the systems are cheaper than a full dimming system, and they will achieve higher levels of cost savings than standard on/off.  Disadvantages to the system are similar to that of the switching system.  The lights can be as distracting, and the ballasts need additional wiring with more commissioning.  Bi-level systems are typically installed in factories, gyms, warehouses, and other spaces were non-detailed tasks take place and where switching is not distracted. 

The final option is a dimming system.  Dimming systems gives an unlimited range of control in the space.  The benefits are that the system can be programmed to have an exact FC level at all times, it is the most comfortable for clients, and has the highest energy savings of any system.  The disadvantage to the system is the cost of the ballasts and the wiring of the ballasts as well as the commissioning level required to perfect the operation.  These systems are best utilized in classrooms, laboratories, office spaces, and libraries, spaces that have highly detailed tasks. 

With daylighting becoming more prominent in buildings, Codes have begun to put standards and requirements in place for monitoring the installation of daylight systems.  With California leading the US in Green Standards and design, requirements for ASHRAE 90.1 have been developed in conjuncture with California Title 24.  Daylit areas must have 2 levels of ouput, 0%-35% and 50%-70% or continuous dimming.  Additional lighting power is allowed per code if daylighting is incorporated into various spaces when mandatory and advanced lighting controls are used in the space.   

So the last piece to discuss is what makes a good daylighting system.  When you strip away the parts and pieces, the open and closed loops, the switching and the dimming, the system is really only as good as the commissioning that was put in place to tell the system to operate.  All the dimming ballasts in the world, all the sensors, and all the dimming curves, none of it means anything unless the system is set up right.   

Commissioning an open loop system, a single adjustment setting on a proportional control can be set at any time that daylight is present.  The adjustment should be made when the daylight distribution is representative of what is typical.  You cannot commission this system during a period of time when direct sunlight is streaming in, if that is not a norm. 

Commissioning of a closed loop system becomes a two step process.  Maintained illuminance level is set by making an adjustment with only the electric lights on (no daylight).  Once the level is set, the ratio between the photo sensor optical signal and the desired light level is set by dimming or switching the electric lights until the desired light level is achieved.

Bottom line, make sure you have a licensed commissioning agent commission your system for proper operation. 

So, Daylighting points to think about.  ASHRAE 90.1 2010 will be the first year of mandatory Daylight control requirements.  Options for daylighting system include open/closed loop and switched/bi-level/dimmed solutions.  Benefits of daylight systems vary from energy saving to improved occupant productivity. 

Daylighting is a reality that is fast approaching, and the more you can learn now…the more enlighting you will be later…

Dimming, why are my lights flickering?

March 7, 2011

We were asked to help with the dimming controls on a theater/showroom the other day, and were inadvertently forced to track the history of dimming.  It’s funny, not the issues that can arise, but the different answers that can be supplied by different manufactures.  I suppose it really boils down to viewpoint and whether you want your product to get specified…but that’s beside the point.

The design of the space utilizes multi-circuit track.  Multi-circuit track is awesome in the way it can provide levels of control without forcing the design to split the track into sections; however, the track is the weak point in the design when dimming is introduced in the space.  More specifically, the number of neutrals in the track is the weak point. 

So the place to start at is what is a shared neutral, and why bother sharing it?   Universally, a three phase system is the acceptable method of transmitting and generating energy.  Three separate sources generate a sine wave voltage that is identical in magnitude, but 120o degrees out of phase.  Think of a grid with an array running along the 0o axis, another array at 120o and a final array at 240o.  You now have three phases of power, equal magnitude opposite direction. 

Back to that question, when running an electrical circuit, a typical wiring system has 3 wires: a Hot, a Neutral, and a Ground.  The hot wire represents the path away from the panel with the neutral being the path of return.  When the circuit is completed, the sine wave is generated and travels out from the panel on the hot wire and returns via the neutral wire. 

I know, I still haven’t answered the first part of the question, but I’m about to.  The previous example looks at one phase, and we need to look at three phases.  A standard three phase system, in lieu of 3 wires, has 5 wires: (3) Hots, (1) Neutral, and (1) Ground.  It works under the same general concept, with the hot wires representing the path away from the panel, although the three different paths will all be at separate phases, 120o apart from one another.  To return to the panel, the (3) sine waves will share the (1) neutral wire, which creates the shared neutral.  The waves can share the wire because they are out of phase, 120o apart. 

Why is this done?  Well that’s a much simpler question, cost.  It is cheaper to run conduit with (5) wires as opposed to (3) conduits with (3) wires in it.   Copper, after all, is not that cheap. 

Great, so we now know that electricity is willing to share, but we’re still not up to speed on why this creates problems with dimming.  Remember, we’re talking about non-linear loads when looking at the current supplied to a lighting load.  With non-linear loads, Harmonics are introduced to the equation.  Harmonics are currents that occur at multiples of the power line voltage frequency.  Here in the US, our line frequency is 60 Hz, and we run into harmoics at 60 Hz, 120 Hz, and 180 Hz.  

Harmonics cause irregularity in our sine waves, and in a three phase system, the irregularity is the same across all 3 phases.  This will cause the sine wave on each phase to be manipulated, and the 3 waves will fall out of sync with each other.  On a 1-phase (3) wire circuit, the harmonic is not an issue as it will travel back along the neutral wire.  On a 3-phase circuit, there is a much different result.  Because the 3 separate phases are no longer 120o out of sync, the waves do not cancel one another out, and a current is induced on the neutral wire.  The harmonics, the “noise” on the wire, can cause lamp flickering and noticeable irregularities in the way a dimming system functions. 

Harmonic “noise” isn’t the only negative affect that can occur on the shared neutral.  The third order harmonic current on the neutral is the sum of the harmonic current on all three of the Hot wires.  For three fully loaded circuits, the neutral current can elevate to 3 times what is present on any of the phase conductors.  If the wire is not properly rated, neutral conductor overheating or unexplained voltage drop can occur.   3 times is a worst case scenario.  Typical distortion runs an average of 1.37 times phase current. 

So, how do you avoid running into these situations?  The simplest solution was already mentioned in the previous paragraph.  Provide a dedicated neutral wire to each circuit to eliminate the cross talk and shared harmonics between the phases. 

So now we are back at the beginning, the track, a track with (3) circuits and (1) neutral wire.   Even if you run separate conductors, you are still sharing a neutral inside the track.  When forced to share a neutral, align the fixtures on the same phase.  Please note, that the phasor sine waves will become additive and the neutral wire must be rated to handed the additive value of the current on all (3) Hot wires.  Or, the current must be limited so the max additive current will not exceed the 20A rating. 

Be careful, and practice safe dimming.

Daylighting 101

February 25, 2011

Daylighting 101

Since the inception of LEED, daylighting has become a more integral part of building design. The option of providing daylight into the space to earn a point has left many engineers and designers looking for more ways to get light and views into the space. I plan to give a quick overview of daylighting, talk about ways to introduce daylight into a space, and finally some benefits to daylighting. In a following post, I will explore more into how the daylight system can work, energy code, and what makes a daylighting system really successful.

Let’s start with getting light into the space, as, coincidentally; the concept of daylighting revolves around having day light. The common daylighting techniques involve vertical and horizontal options.

The most common type of daylighting scenario we see (particularly in Washington DC) is vertical windows. All buildings (except the occasional top secret ones) have windows. Typically called side lighting, providing vertical glazing on a building introduces both daylight to a space, as well as an exterior view to the space. Vertical solutions are supplemented with additional systems to introduce light deeper into the space. Light shelves, open office space, and glass walls are architectural solutions that help bring daylight deeper into the space.

Less common solutions to introducing daylighting into a space, are the horizontal options. Horizontal glazing will typically provide larger amounts of light into the space, particularly the interior spaces, but it limits the opportunity for exterior views. Horizontal lighting introduces daylight with less manipulation of the common architectural elements, but with more extensive design of the ceiling and roof top spaces. Skylights and Light pipes are two common solutions for horizontal lighting.

Great, so we’re bringing light into the space from either the vertical windows or a roof top solution. But what do you have to worry about? Well the number one thing you have to worry about is the sky. Depending on your location, the common sky condition will be a clear sky, a partly cloudy sky, and an overcast sky. You cannot depend on sunlight. It will vary based on your location and based on the time of day. Sunnier cities receive a higher ROI on their systems. The last thing to remember about the sky is how the clouds will affect the dimming system. Fade time is the length of time between the photosensor sensing the lack of sunlight and adjusting the lighting levels. Areas with quickly passing clouds need to have a slow fade time, while areas with slowly passing clouds can have quicker fade times.

One of the harder daylighting considerations for the engineer to control is the building orientation. Coordination with the Architectural team prior to building design can help orient the building in the most beneficial location. The buildings length should be located on the east/west axis while the area with the primary daylight harvesting systems being exposed to the North. Of course, you also want to make sure that your daylighting system isn’t going up at a window that has a giant tree on the other side of it.

The final concern for daylighting is the effects of the sun. While the sun is providing the light that we want to harvest, we want to avoid harvesting the harsh direct light from the sun. Adding daylight into the space while not accounting for solar heating gain can severly tax the HVAC system, and cause a temperature discomfort along with the daylight gain. Coordinate specification of windows with a low solar heat gain coefficient, or install a system that eliminates the harsh direct sun light.

So why daylight? What can you possibly get out of a system?

The first argument the owner wants to hear is payback, or more simply, how much energy are you saving me? Keep in mind, the results I’m about to give you are based on a building designed for daylighting. The space has glass walls, low partitions, and a system set up with two daylighting zones (first zone is 10’, the second zone is 10’ to 30’). In the study, performed by Lawrence Berkeley National labs, the energy saving was between 30% and 60% with the second zone saving 10% to 40%. Done right, daylighting can be a powerful tool.

But daylighting can bring more to the table than monetary savings. For one, having a building with a daylighting system sends the correct corporate message of sustainability and a commitment to new technology. Day lit spaces also create an incredible psychological benefit for the people using the space. People tend to be happier and work more efficiently in spaces with large amount of daylight. For a company, the salary of its employees is typically the largest cost associate with the company, and a happy more efficient work force can go a long way in justifying a daylit space.

Stay tuned for part two, and feel free to leave any questions below.

Interference between AWG and CAT 5

February 18, 2011

Interference between AWG and CAT 5

 (Hard facts courtesy of Marcus Ernst, Humor by Brad Hartman)

 One of the issues with buildings is that there are a large number of systems in a building that don’t play well together when they are mixed.  Two of the systems that tend to fight on the playground are electromagnetic (power lines, fluorescents, etc.) and radio frequency (multiple transmissions on the same wireless frequency). 

 So with both systems occurring in buildings, how do you protect the systems from interference?

 Not as simple as just asking it to stop.  There are 2 types of interference, radio frequency (RFI) and electromagnetic (EMI).  CAT 5 cable is designed to mitigate RFI due to the fact that the wires are in twisted pairs.  Electromagnetic interference is typically eliminated by using shielded cable.  The shield, aka drain, is typically grounded at 1 end to remove the EMI charge.  CAT5 does not have a shield as it also known as UTP cable (unshielded, twisted pair). 

The cables are designed, or cladded in a way to minimize the interference.  To ensure that EMI is not an issue, you can put the wires feeding the fixture in question, in EMT and use compression connectors and couplings.  This way there is a good bond between the sections of EMT.  Also, make sure that the EMT is grounded at one end.  Any stray EMI will be captured and grounded.  At this point you have eliminated any RFI and EMI issues. 

As a general rule, you should try to maintain a separation between line and low voltage (data) of a minimum of 6”.  All crosses should be done at 90 degree angles.  This is given the typical construction materials used MC and EMT with set screw connectors and couplings.  When this is not possible, bonded, grounded EMT will serve to mitigate the EMI.

Adding Lighting Control to Emergency Circuits

February 14, 2011

The question was posed to me about emergency power and when a bypass device would be required in lieu of a transfer device.  From this question, the second natural questions to ask was how the devices work.  The first question, I’m going to refer you to code (NEC, UL 924, Emergency).  As far as how the devices work, the following discusses how the following devices work: Shunt Relays, Emergency Bypass, and Emergency Transfer. 

 Shunt Relay

 Often time, designs have dimming, switching, or control levels in rooms or zones with emergency requirement (i.e. conference rooms).  The intent of the shunt relay is to provide a level of lighting control on an emergency circuit, while still allowing the circuit to switch to 100% ouput during power failure.  Shunt relays are a bypass device. 

 Emergency Shunt Relay (Detail provided by LC&D, click here for information on the shunt)

Emergency power is fed into the shunt relay, connected to one end of a normally open relay, and then continued out to the switch/dimmer/ relay before being connected to the load.  A wire is also run from the second side of the normally open relay, and is connected to the first set of wires downstream from the control device and upstream from the load.  This second wire forms the bypass route for the emergency power to take during failure, while the normally open relay forces current through the control device up until the moment of failure. 

 The shunt relay is also connected to normal power; however, this connection is designed only for power sensing purposes.  Once the shunt relay is connected to normal power, it will hold the normal open relay open until power lose is detected on the normal circuit.  At the moment of power lose, the relay will close, and electricity will natural flow down the path with least resistance, the path without the lighting control.  At this point, lights on the emergency circuit will turn to 100% and will remain at 100% until power is restored to the normal circuit, which will cause the relay to open, forcing power through the lighting control. 

 This solution is typically seen in conference rooms requiring emergency lighting.  The zone designated to turn on in the event of an emergency will be powered by an emergency circuit.  The shunt relay will be connected prior to the device controlling the lighting zone, and the zone will operate according to the control device until power loss is detected. 

 Emergency Bypass

 Emergency Bypass takes the Shunt Relay one step further.  The device is still positioned to direct power around a switch or control device.  However, with the transfer device, the normal power feed is providing power to the light, and the device will transfer the source to emergency power during failure. 

 Emergency Bypass – (Detail provided by Nine-24, click here for more information on the transfer)

When looking at the diagram, you will see it looks a little different.  The Black, Red and White are the Normal power connections.  White is neutral, and the black and red are normal power.  The black connection is uninterrupted, while the red connection has the switch, photocell, and time clock tied to it.  The red wire is providing the power to the lights during normal operation, and the power will be limited by the devices downstream (it will dim with the photocell, off/on with the switch/timeclock, etc).  The black wire is there for the purpose of sensing, similar to the normal power connection on the Shunt Relay.  During normal power operation, power flows from the source, through the control device, down the red wire, and out the yellow wire to the load.  The circuit is completed from the load, back to the white/blue wire, and then out again from the white wire back to the panel. 

 The emergency power is connected by the blue, yellow, and white/blue wires.  Blue provides the emergency power into the Bypass, and yellow takes that power out to the lighting load.  The white/blue wire is the neutral path from the lighting load back to the normal power during normal lighting operation, and it is the neutral path back to the emergency power during emergency power. 

 When the Bypass devices does not detect power on the black normal wire, it means there was some form of power failure.  At that point, inside the bypass device, the device will transfer the connections, and power is provided from the emergency source by the blue wire, and out to the fixture from the yellow wire.  The circuit is completed from the load back to the emergency panel (it will not follow the white/blue wire back to the device). 

 In this situation, the control device is avoided and the lights will turn to 100% output.  The lights will remain at 100% until the black wire detects normal power restored, and the transfer device will reroute power back through the control devices in the room. 

  Finally, Emergency Transfer

 Emergency transfer, like the shunt relay and bypass device, is designed to allow lights to turn to full output upon power failure.  The transfer works the same way the bypass device works, in that it takes a normal and an emergency input to the device.  However, it transfers the hot wire and the neutral wire for full emergency control. 

 Emergency Transfer (Detail provide by Nine-24, click here for more information on transfer device)

The detail provided above looks a little different from the last two, but don’t let the number of connections confuse you.  The Nine-24 device provide additional contacts to allow input from different systems.  In the detail above, a Fire Alarm Panel is shown.  In this case, a signal from the fire alarm system could cause the lights to switch to 100% output rather than waiting for the Transfer device to detect power failure. 

 The lighting control is shown on the top left portion of the detail.  Power is routed from the normal power panel , down through the dimming cabinet, and then into the BLTC device (note in this case it is labeled as a dimming cabinet, but this can be any type of lighting control).  The normal circuit is tapped prior to the dimming cabinet and run into the top portion of the BLTC.  This connection serves as the power sensing portion of the device, and will initiate the transfer to emergency power during power failure. 

 Emergency power is fed into the device, and the hot and neutral feeds are connected to the same ports the normal hot and normal neutral feeds were connected to.  With the dual feeds connected to the transfer device, when power failure is sensed, the bypass will transfer the normal hot and the normal neutral to emergency hot and emergency neutral.  Lights fed downstream from the BLTC device will turn to 100% output. 

In summary, the Shunt Relay, the Emergency Transfer, and the Emergency Bypass device will all allow a controlled lighting zone to turn to 100% output during a power failure.  A shunt relay allows a level of control to be applied to an emergency circuit, and the Shunt and Bypass transfer power from a normal source to an emergency source with the transfer only transferring the hot wire, and the bypass transferring the hot and neutral wires.  The different devices are required for different occupancy levels per Code Requirements, and you should verify the type of emergency control that is required on your project prior to specifying a device.   

 Bonus – How do you turn a light on by turning the switching off? 

 Each of the override devices have a sensing element associate with the device.  When connecting the device to normal power, if the sensing leg is connected to a switched leg, the device will think power was lost any time the switch is flipped to the off position, and the emergency light will be turned to 100% output.  Turning lights on, by switching them off.