LCDI cord sets have individually shielded conductors, and a circuit in the plug that detects current through the shields and cuts off the power. They are meant to detect live-to-neutral current within a damaged appliance cord. They do so faster than the circuit breaker and at lower current. (A breaker pops at 15A or 20A but the threshhold for undesired current within a power cord is 0A so the LCDI can be a lot more sensitive). GFCI devices do not detect live-to-neutral current within a cord because they function by detecting live/neutral current imbalance.

Newer window air conditioners are required to be equipped with LCDI cords, apparently because the pattern of usage of window A/Cs tends to cause cord damage. They are moved from room to room or moved to storage during the winter and this can cause cord damage that can cause fires. So says a Google search, including various superficial documents from NEC and UL.


Why only window A/Cs? What I don't understand is the background to the requirement that makes this a good idea for window A/Cs, but not a good idea, or even a better idea, for kettles, table/floor lamps, guitar amps, laptop computers, etc ... anything that A) gets moved around or handled a lot and B) is left plugged in and unattended most of the time.

There are some contrasting cases, like say a vacuum cleaner, that is subject to huge abuse but isn't left plugged in all the time so if the cord catches fire the user will notice in good time.

It does not seem to me like window air conditioners would be the poster child for undetected cord damage. And yet, they are the only appliance where LCDI is required. Why?

  • 3
    NEC 440.65 allows an LCDI, and AFCI, or a HDCI (Heat Detecting Circuit Interrupter). Note that Part VII 440.60-440.65 is a section of code solely about Room Air Conditioners. The obvious answer is that there have been enough fires or shocks from room air conditioners that it was thought necessary to add such protection. Generally speaking, code is written in blood.
    – Jon Custer
    Dec 1, 2022 at 14:59
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    My guess is that the fact that window air conditioners get rained on plays a part in this. The fact that there are often radiators under windows, which are usually grounded, may also contribute. Agreed that the ultimate answer is "someone noticed a pattern of people getting hurt by this specific situation". Note that these days hair driers often carry this protection. @JonCuster, I think that should be an Answer..
    – keshlam
    Dec 1, 2022 at 15:29
  • @keshlam Hair dryers are protected by GFCI, which does not protect from the specific kind of cord damage that LCDI is designed for. Wetness in window A/Cs, from rain or from their own condensate, may create current paths to ground but those are not protected by LCDI circuits.
    – jay613
    Dec 1, 2022 at 15:42
  • I have cousins that do construction and their understanding is that GFCI/RCD plugs are "wet outlets".
    – dandavis
    Dec 1, 2022 at 17:23
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    Thanks @isherwood. Not questioning your privileges or the value of your votes! Just appreciative to hear the background.
    – jay613
    Dec 2, 2022 at 20:46

3 Answers 3


I believe this is the failure scenario for portable AC units:

  1. The cord is routed through a window frame or door and gets smashed. Or it gets routed in an area where it gets stepped on. The cable gets flexed during handling.

  2. Conductor strands get disconnected due to this damage. The current-carrying capacity degrades as the conductor resistance increases.

  3. Air conditioning is a high-power load used for long periods unattended. The loss of ampacity causes the conductor wires to heat to unsafe levels and the insulation melts.

  4. (LCDI protection) As the insulation melts, the conductor is exposed to the LCDI shield and shield current is detected. The protective device trips and power is removed. The damage is limited to a burnt spot.

  5. (AFCI protection) As heat builds, the copper wire melts, causing a arc in the line, which is detected, causing a protective device trip. Again, damage is limited to the cable.

  6. (No protection) Heat continues to build, degrading all other insulation. A live-neutral arc forms, generating large amounts of heat, igniting the rest of the cable and anything around it. Your house burns down.

Why can't GFCI be used? Note the requirement in NEC 440.65 is for single-phase units. Here, the neutral wire is nominally at ground potential, but carries current, so it may not cause significant current to flow with a grounded shield.

Considering other devices: A permanently installed air conditioner has exposed cables protected with conduit, preventing damage.

Electric vehicle charging has a very similar protection. At 240 V split-phase, both line wires are at a potential to ground, so the shield can be grounded and GFCI used. For DC fast charging, the lines are isolated with a transformer, so resistance to ground can be used as well. There are also temperature sensors at the vehicle plug.

As mentioned by the OP, vacuum cleaner is a high power load with a cable subject to abuse. However it is always attended when in use, so melting and fires can be detected early. A hair dryer is similar; the hazard there is electric shocks with water, so it has a different means of protection (IDCI or GFCI).

An electric clothes dryer or stove is a huge power consumer, but is rarely moved after installation and utilize purpose-installed outlets, so the cables don't run in vulnerable locations.

Cooking appliances like a microwave or toaster are again attended in operation and are run for minutes, not hours per day.

Home electronics like a TV or PC are low amperage, so cable damage results in less heat.

So a window AC is a home appliance that is plugged into arbitrary outlets with a cord, drawing large amounts of power, while unattended or while people sleep, causing a unique fire hazard.

  • So you're saying I'm too focused on one-step failures where the cord is damaged enough to cause live-neutral leakage but not enough to cause a full blown short. That would be shared by low-power appliances. But the A/C-specific scenario is a two-step failure where slight damage causes heating which subsequently causes further damage, and that is unique to high-power appliances? Hmm. That explanation works, I mean it's plausible. Is that really what happens to window units all the time?? IE, that's the "blood"? If so, I can't think of another appliance that matches the circumstances.
    – jay613
    Dec 1, 2022 at 22:15
  • @jay613 Certainly live-neutral arcing has been a big enough issue to require panel-box AFCIs in nearly all indoor circuits now. But I think there's also factors like a live-neutral fault requires compromise of two layers of insulation (which admittedly could happen in one damage event), and traditional PVC cable insulation is much more pliable and doesn't fatigue like copper conductors.
    – user71659
    Dec 2, 2022 at 1:32

LCDI is GFPE for air conditioners

The reason window air conditioning units have LCDIs is because a) they are high-power, long-running loads that benefit from the fire protection provided by residual-current (differential) protection but b) cannot use a straight differential protector due to excessive operational leakage.

The protection from fire due to wiring damage provided by differential protection is well-known, so we won't elaborate on it here. However, the answer to "why not just put ground fault protection in place?" has to do with why this only applies to air conditioners: hermetic refrigeration compressors are known sources of leakage current to the earthing system. This operating leakage increases as the compressor winding insulation wears, and is often quantified as part of the PM routines for industrial compressor units in the form of regular "megger" tests of compressors. (It's also why old refrigerators trip GFCIs.)

Hence, the LCDI uses a shield mesh in the cord to "pick up" current leaking from a damaged cordset instead of relying on differential protection alone to do the same.


UPDATE: My basic premise of "cost/benefit" and most of "why specifically window air conditioners" still applies. But after some helpful comments from @jay613 and watching this teardown video I have a much better understanding of exactly how an LCDI works. It monitors for leakage current using a sense wire but does not specifically check for the same problem (a ground fault) that a GFCI tests for. I have a hunch (but no time to research at the moment) that a circuit GFCI (e.g., GFCI/receptacle or GFCI/breaker) will in fact trip under many, but not all, scenarios where an LCDI would trip. More later, when I get a chance to research a little more. For now, I will strike-through the parts that I expect to change.

I believe it comes down primarily to cost/benefit analysis and the type of usage of window air conditioners specifically.

There is a cost to almost any technical safety improvement. Over time those costs go down, but initially they are quite significant. This is certainly the case with GFCI and AFCI protection. So there is a balance between cost (how much to install in every necessary location) and benefit (how many fires, injuries, deaths (yes, lives have a cost in the world of law and government regulation), property damage, etc.). If it was not for the cost:

  • GFCI would have been mandated for all areas with water ingress concerns as soon as reliable devices were available - i.e., not just kitchens and bathrooms but all the later areas - laundry room, unfinished basement, garage, etc.
  • GFCI and AFCI, as appropriate, would be required with every panel upgrade, circuit change, etc. Grandfathering older stuff slows down general safety improvements but at a major cost savings - and in fact if the costs were added in then many otherwise very important safety upgrades might not get done, or would be delayed until there was no other choice.

There are certainly many other examples, and not just in residential electrical but in automotive engineering and other fields as well.

An LCDI costs more than a GFCI (actually, not sure - I suspect they did initially but with economies of scale a plug-LCDI or a plug-GFCI is likely comparable cost) which costs more than a simple cord and plug. So GFCI gets mandated (many years ago) for the most critical things - hair dryers and other appliances used by wet people around bathroom sinks, and later gets added to some more appliances but never, due to cost, gets added to low-risk appliances - lights, battery chargers, TVs, computers, etc. The high total cost for adding that $5 GFCI to all those devices that have extremely low risk of being used in a ground fault situation, combined with increasing implementation over time of GFCI protection in the power supply (GFCI/receptacles and GFCI/breakers) means that it just doesn't make sense to do that. All the more so with an LCDI that costs more than a GFCI (the device itself is more complex and it requires a more expensive cord).

Window air conditioners are a special case where LCDI has additional benefits:

  • They produce water. Few other household devices do that (dehumidifiers). People are plenty aware of (after many years of GFCI implementation in kitchens and bathrooms) of the dangers of "water + electricity", but many don't think about that when it comes to a device (air conditioner) that is not used with water (like a coffee maker or washing machine) and not used near water (like a toaster in the kitchen or a hair dryer in the bathroom). Actually, they do produce water, but that isn't what LCDI deals with.
  • They use a lot of power. HVAC in general (window air conditioners, space heaters) are the largest plug-in devices for most people that run for more than a few minutes at a time. (Kitchen built-in appliances may fall in that category, but in most cases if not hardwired then the cord/plug is hidden so it doesn't get moved around and it doesn't even get touched by people in normal use.) No update here - because they use a lot of power, a cord problem is still a big issue.
  • Many, but not all, users will move the entire unit twice a year with the change of seasons, which lends itself to cord/plug problems much more than with something like a refrigerator. Again, no update - still applies 100%.

and what I suspect is the biggest factor of all:

  • Window air conditioners are used in the oldest buildings. If a building gets a major renovation, HVAC upgrades are usually part of that. For new small buildings (shed, workshop, separate garage), a mini-split makes sense. New larger buildings (typical single family homes) have generally had air conditioning built in for decades, going back to before GFCI was even generally available (let alone required). Even many older homes that did not have central air conditioning when built had forced air heating so adding central air conditioning was relatively easy to do. But window air conditioners tend to be installed:

    • In very old housing (especially 1940s and older) which has had only minimal electrical upgrades (e.g., a new circuit for the air conditioner - maybe, if the landlord can get away without it then not even that) and no (or few) GFCI-protected circuits. Even if owner-occupied, the cost to add central air conditioning is very high so that upgrade will fall behind relative to other more critical problems, which happen a lot with older homes.
    • In rental housing with low-income tenants - the landlords only put in upgrades required by law and the tenants can't afford to do any upgrades themselves
    • In rental housing many tenants consider themselves short-term (e.g., college students) so that even if they have funds available they are unlikely to put them into any major upgrades since the upgrades would revert to the landlord when they leave

So window air conditioners present a unique combination of water (yet without being connected to the plumbing system), high power, movement and installation in older buildings.

  • 1
    I've done quite a bit of reading on this in the last few days and I think I get it. So I say this respectfully, but I don't think LCDIs protect from water-related ground faults, I don't think they protect from any kind of overcurrent, and in fact I think they work even if they appliance is switched off. That's actually the point of my question. My understanding is that LCDI cords do not include GFCI protection, they are not "GFCI+LCDI". ... (1/2)
    – jay613
    Dec 1, 2022 at 15:50
  • 1
    (2/2) So your key points, A) water and B) high power, are I think irrelevant, and your point about movement compared to a fridge side-steps my question, which compares instead to things with A/C power cords (not wall warts) that DO get moved around a lot, like kettles, laptops, guitar amps, etc. And IMO the power cords that are MOST left plugged in all the time and MOST apt to damage, floor lamps.
    – jay613
    Dec 1, 2022 at 15:52
  • 1
    I agree it's something to do with cost and blood. You want to minimize both. I get that ... I just don't understand the specific focus on this one appliance.
    – jay613
    Dec 1, 2022 at 15:53
  • 1
    I agree with @jay613, none of this really explains why air conditioners specifically need LDCIs while things like space heaters don't.
    – nobody
    Dec 2, 2022 at 1:38
  • 1
    Good link to video teardown in your update. Interesting that what triggers the solenoid is not current but voltage, specifically contact between the live wire and the braid. It doesn't change the question or the answer but if my interpretation of the video is right, I'll need to change the "background" explanation in the question.
    – jay613
    Dec 2, 2022 at 15:05

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