Talk:Residual-current device

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[edit] GFI/GFCI

From the article:

The terms ground fault circuit interrupter (GFCI) or ground fault interrupter (GFI) are used in the United States and Canada, but these terms are not, strictly speaking, correct (or at least complete), because a RCD will trip if current leaks anywhere, not just to ground.

In NEC terminology, power flows between the "ungrounded conductor" and the "grounded conductor" (with the load in the middle) . If more power is flowing through the ungrounded conductor then the grounded conductor (unbalanced), then there is a ground fault (a failure to ground properly), and so the circuit should be interrupted. Looking at it this way, the terms GFCI and GFI make more sense, and is how I always assumed they came about.

Notes: The "ungrounded conductor" is colloquially called "hot" or "line". The "grounded conductor" is also called the "identified conductor", and is colloquially (and sometimes incorrectly) called "neutral". Note that none of these are the "equipment grounding conductor", which is colloquially called "safety ground" or just "ground". Yes, NEC terminology is confusing.

--DragonHawk 15:23, 19 Jun 2005 (UTC)

But ground isn't the only place the errant current can go; it might return on the other hot leg of a split-phase system, the neutral leg of another branch circuit, or it might even leak unto a de-energized part of another circuit (the switched leg going to a lamp or a circuit de-energized by opening its breaker). I think it was this thought that provoked the original author to speak of "ground fault" being something of a misnomer. And it's certainly true that the devices sense "balanced current" and not a "ground fault" in particular. Compare this with certain ungrounded systems (for example, railway traction motor circuits) where there ctually are devices that are purely sensitive to ground faults; any grounding of the ungrounded system trips the detectors and removes power. Such devices would be properly called GFCIs.

Atlant 11:55, 20 Jun 2005 (UTC)

Yes, and also in many renewable energy systems, some of them are wired up in a balanced way with neither of the two wires connected to ground, in which case, of course, there must be a breaker on both sides. In this case there is no ground, and no concept of ground fault. Thus what's really being measured and detected is a balance fault, i.e. basically what would be a violation of Kirchoff's current law, were it not for the unknown existence of some other leakage current somewhere else (not necessarily to or from ground). Also the concept of a balance fault nicely generalizes to three phase, where, for example, a three-phase BFI could trip if there was any imbalance of total current (i.e. if the total current didn't sum to exactly zero). Glogger 23:02, 30 December 2005 (UTC)

I have (re-?) inserted a brief explanation that GFCI is the usual name in the U.S. and Canada, which it certainly is, regardless of arguments as to whether the term is strictly defensible. On a related note, is "Balance Fault Interrupter" used anywhere? If it does appear in documentation or industry literature, a cite is in order; if it's new coinage proposed by a Wikipedia contributor, it really doesn't belong. Sharkford 19:27, 1 February 2006 (UTC)

Strictly speaking, a GFCI is different from a RCD in that a GFCI will trip in the presence of a neutral-earth fault (even though there is no voltage difference). It does this by having a second coil around the supply conductors (as well as the sensor coil). This provides a positive feedback loop when the neutral-earth circuit is made (ground-fault) and the GFCI trips. A RCD will not do this.--210.55.171.62 23:14, 17 October 2006 (UTC)

[edit] miswirings

From the article:

Manufacturers are now designing such devices that will disconnect their circuit, even if they don't dectect electrical leaking, if they are wired improperly.

Really? How? Under all possible miswirings, or only some? -- The Anome 08:50, 26 May 2004 (UTC)

I don't understand this either. It's just a guess on my part, but since the most common wiring error is reversal of line and load connections on receptacle devices, the manufacturers may be doing something to detect that particular condition and trip. UninvitedCompany 14:44, 26 May 2004 (UTC)

Some types of RCD incorporate an additional device which trips if the voltage between the earth (ground) and neutral conductors rises above a certain potential. This would also detect a miswiring where the live (hot) and neutral conductors were reversed. --Ali@gwc.org.uk 13:20, 17 Jul 2004 (UTC)

[edit] mixing mains and signal wiring in us

I'm concerned by the comments at the bottom that suggest that, in the US, you can intermix line voltage wiring with Ethernet, etc. I'm not aware of *ANY* US code-approved wiring solution that allows that, although I suppose a partitioned mounting box might allow the use of a single Decora faceplate with, say, line voltage components on one side and low-voltage components on the other side.

Any NEC experts here to say yea or nay?

Atlant 18:25, 13 Dec 2004 (UTC)

Hubbell makes a variety of products that put ethernet, phone, and power into the same faceplate. Of course it's nicely designed to keep things separated by metal dividers. From an aesthetic and design point of view, it's fantastic, the way they got everything lined up, so when you install all the sockets you have "all your ducks in a row" so to speak. Glogger 23:07, 30 December 2005 (UTC)

Yes, the low-voltage stuff is either on the other side of a divider or, commonly, outside the box altogether, but sharing a common (large) faceplate. I have edited this section and simply deleted the refs to low-voltage wiring; it's not particularly common and has no special relevance to this article. Sharkford 19:12, 1 February 2006 (UTC)

[edit] RCD vs RCCB

whilst the term RCCB is more descriptive it seems to have fallen out of favor at least here in the UK. Nearly every manufacturer and most standards and guides use the term RCD nowadays. Should we change the article to reflect this (if we do we should probablly also do a pagemove). Plugwash 01:51, 30 Mar 2005 (UTC)

My taste would be to:

Create a redirect from RCD to RCCB
Modify the lede of the article to mention the new term and how it is replacing the old term

But I don't feel strongly about this; if you'd rather move the article, flip all the references from RCCB to RCD, and add a bit to the lede describing the now-archaic ( :-) ) term RCCB, feel free! But don't forget to clean-up all the resulting double-redirects of RCCB that may exist out in Wikispace. (It's for this reason that I generally don't rush to rename an article whose name is still within the bounds of "accurate enough".)

I assume that the lede also mentions GFI, which is the universal, casual term in the United States. If it doesn't, I'll edit it in right now. (It's okay; GFI and GFCI are both there in the second 'graph.)

Atlant 12:16, 30 Mar 2005 (UTC)

[edit] Arc-Fault Interrupter

Here in the U.S., we're starting to see circuit breakers that are able to sense arcing current. (I don't know exactly how they do this, but I'd assume it depends on sensing high-frequency noise in the mains current waveform.) I think this capability is often combined with Balanced-Current fault detection.

Does anyone feel ambitious enough to investigate this and either 1) create a AFI redirect and modify this article to include the concept or 2) create a free-standing article with appropriate cross-references to here and back?

Atlant 12:18, 30 Mar 2005 (UTC)

Aha! I found the pre-existing article and linked it as a "See also".

Atlant 19:03, 30 Mar 2005 (UTC)

[edit] How do RCDs actually trip?

Using the example of the illustrated "outlet-style" RCD, the description used to speak about the solenoid being continuously energized and that providing the latch that kept the RCD "reset". This description is clearly inconsistent with at least US-style RCDs as it would imply that a power failure would also lead to the dropping-out of all of the RCD solenoids, their subsequent tripping, and the need to manually reset all the RCDs after power was restored. This ain't the way it works in practice.

I conclude from this that the mechanism operates in the opposite fashion: The latch is mechanically held and the RCD activates the solenoid when it wants to "trip" the RCD by unlatching the latch. I've changed the text to reflect this view of the world. But if Europe has "outlet-style" RCDS and they operate as originally described, please feel free to extend the text to explain this.

Meanwhile, I guess I'll take an American RCD apart this week-end and check for certain. :-)

Atlant 19:27, 3 Jun 2005 (UTC)

In Europe you can get both passive rcds (which don't trip out on power failure) and active rcds (which do). Generally most rcds in consumer units etc are the passive type but the active type is considered advisible for supplying equipment which can pose a danger on unexpected re-energistation. Plugwash 20:57, 3 Jun 2005 (UTC)

Thanks! Atlant 23:22, 3 Jun 2005 (UTC)

[edit] Leakage limits: real safety or imaginary?

"Residual current devices (RCD) or residual current circuit breakers (RCCB) are circuit breakers that operate to disconnect their circuit whenever they detect that current leaking out of the circuit (such as current leaking to earth through a ground fault) exceeds safety limits."

Safety limits are not really what its about. A typical example that shows this is a damaged immersion heater. It is very common for the element casings to split open allowing electrical power to flow directly through the hot water. This is a normal part of the life cycle of an immersion heater. This will trip any RCD immediately, although (counterintuitively for those without much electical knowledge) there is no danger, ie people are not getting electrocuted because of it. RCDs tripping due to split elements of fixed appliances does not in reality give any added safety, thus is more correctly described as a nuisance trip. I realise this may sound counterintuitive, but bear in mind safety is removal of danger, and danger equals number of deaths, and split immersion elements are simply a non issue from a safety POV. They arent electrocuting anyone.

How is the 30mA figure reached? It is primarily a question of what will generally stay working. Anything more sensitive will be troublesome, 30mA is mostly ok... though not always. Some installs are horrors from the point of ongoing nuisance trips.

I hope RCDs turn out to reduce deaths, I really do. But... the usual wondrous descriptions of them are I fear significantly inaccurate.

Tabby 21:46, 8 Aug 2005 (UTC)

[edit] Are they really safer?

The article says RCDs increase safety: to be honest we have yet to see. Although they offer protection against known issues, they also introduce a number of dangers.

Firstly users often take full advantage of the protection they offer, using equipment they would never dare before they got their RCD. Examples often seen are plugging wet electrical goods in, using power tools outdoors in the rain, using or installing goods that are known to be problematic safetywise, on the basis the RCD will protect them, and so on.

Also RCDs will kill lighting ciruits quickly in a fire, due to smoke and carbon caused leakage, and deaths are known to have occurred because of this. I curently recommend it is best not to put lighting on an RCD, except with TT installs where they are necessary for basic safety.

Also users tend to believe RCDs will proetct them against risks they dont, and become over confident as a result, exposing themselves to danger. A classic example is that of fitting an RCD plug to an electric drill, in the belief that the power will be cut if they drill into a live wire. In reality no such protection is given by this arangement. First the RCD only detects current imbalance _in the drill's supply_, so it wont even notice the user getting fried, as the shock current is coming from the wire buried in the wall. Second, if by some miracle it did manage to receive a heavenly missive telling it Jo's getting fried, cutting power to the drill, which is all it can do, would make no difference, since the power to the drill case is coming from the wire in the wall, not the drill's own lead. The problem here is simply that the user thinks the RCD will protect them against drilling live cables, ceases worrying about it, and does things that are a threat to life.

I'm not taking an anti-RCD stand here, just being realistic enough to say that the safety benefits are not what is being widely claimed. Or even close.

Only time will tell where the balance lies between the hazards they create and the hazards they protect against. Lets just hope theyre the right move.

Tabby 21:46, 8 Aug 2005 (UTC)

[edit] RCDs and immersion heaters

Tabby writes:

I'm afraid you're not considering all the possibilities. Let's take, for example, my hot tub. It has this nice immersion heater that is in intimate contact with water that's just loaded with ions that make the water very conductive. When the immersion heater eventiually fails, the water is connected to the live windings of the immersion heater and becomes charged to some arbitrary voltage (depending on where the leak in the immersion heater occurs. Typically, they fail near the top end so in the US, you could almost count on the water being connected to a full 120 vac at the end of the split phase-wired immersion heater. (And if you don't believe me, I'll show you a heater that failed exactly as I'm describing!)

Now, step into my hot tub. Without an RCD, YOU probably form the lowest-impedance path from ground to the energized water. You've just used up all nine of your lives, Tabby. But with an RCD, the water conducts enough leakage current to the grounded pumps and such that the RCD trips and de-energizes the water.

The same sorts of situations can happen in houses with electric hot water heaters and one or more runs of plastic insulative piping. Without an RCD, you can get energized water (although the situation may be less severe than the hot tub because the poable water may contain fewer conductive ions). (By the way, are you familiar with the fact that hot water heaters usually install dielectric joints in the copper piping? So even with copper piping, it's not impossible that the copper somewhere in the house won't be well-grounded.)

RCDs save lives, and properly designed RCD circuits typically don't put emergency egress lighting on the same RCD device (or even any RCD device) as the circuits that pose shock hazards.

Atlant 00:16, 9 August 2005 (UTC)

[edit] How to Evaluate Risk

Atlant:

I'm afraid you're not considering all the possibilities. Let's take, for example, my hot tub.

Every imaginable combination of faults and design errors is possible. We could sit here and think up scenarios by the score - but sooner or later one notices that some of these scenarios are common, some rare, and some simply dont happen IRL. Thus what scenario we can think of is not what determines level of risk.

What does determine level of risk is how many people are dying in real life from these imagined scenarios. The one you pose here is not one that is hapenning here in the UK, thus, by elementary logical deduction, it is not a significant risk.

Real world risk is best determined by looking at the facts, not by imagining what scenarios might or might not happen. Its a common mistake.

Tabby

Tabby 22:50, 15 Sep 2005 (UTC)

[edit] Re RCDs saving lives

There is no question that RCDs have saved lives. It is also clear that RCDs have cost lives, and will continue to. The questions are: 1. which number will be bigger over the many years to come, will they save more or kill more? 2. could the cost of millions of RCDs be better spent elsewhere, eg on non slip stair treads? Stairs injure and kill lots of people, electrics dont (in UK).

Tabby

PS I agree with your comment that escape lighting should not be on RCD, but here in UK is routinely is. We have an ongoing history of mandated whole building RCDs too.

Tabby 22:51, 15 Sep 2005 (UTC)

[edit] What is the "residual" in RCD?

I accept the points of those who say that ELCB is not technically accurate but at least it is descriptive. I haven't been able to figure out what "residual" means. Any ideas? Harmonitron

THe residual current is the difference between the phase current and the neutral current. There shouldnt be any diff if you have no leakage to earth. But if there is a leakage it means some normally earthed part is live, and this will be sensed by the balanced coil and trip out.--Light current 23:51, 23 April 2006 (UTC)

[edit] How does earth leakage circuit breaker differs from residual current circuit breaker

IMO it doesnt--Light current 23:48, 23 April 2006 (UTC)

[edit] Disadvantages of RCDs?

I understand from the comments above why RCDs shouldn't be used in lighting. Why are they not used everywhere else? I suppose they cost slightly more and that the differential transformer uses up some extra energy. Are there other downsides? --Stereo 12:42, 6 April 2006 (UTC)

Nuisance tripping!--Light current 23:52, 23 April 2006 (UTC)

Nearly all appliances leak some current to earth and this can seriously add up. For this reason and due to the huge disruption of a trip covering a lot of equipment its inadvisable to put too much kit on the same RCD. However RCDs suitable for fixed installation are still quite expensive (it's a chicken and egg thing, RCDs suitable for fixed wiring won't get cheaper until more people use them and more people won't use them until they get cheaper) so the typical home (at least here in the uk) ends up with a split load board (some circuits only on the main switch the rest all on a single RCD). So a comprimise ends up being made between the desire to have RCD protection and the desire not to load too much on that one RCD. Plugwash 14:23, 5 May 2006 (UTC)

Because of the existence of the combination RCD/duplex-outlet, current American practice seems to be tending towards lots of RCDs protecting individual portions of branch circuits. So, for example, in my house, I have the following RCDs:

One in the garage, protecting all four garage outlets and one associated exterior outlet
One in one bathroom, protecting just that bathroom
One in another bathroom, protecting that bathroom and two others
One protecting a portion of the basement workrooms, the central vac, and two exterior outlets
One protecting one exterior outlet
Three protecting kitchen convenience outlets.
One serving the kitchen garbage disposal
One serving the dishwasher

With this arrangement, nuisance tripping is minized as is the scope of the failure if an RCD trips. With the exception of one kitchen appliance that trips one RCD (which is apparently sensitive to induced noise; there's no ground fault in the appliance.), we hardly ever see a nuisance trip although we've seen some genuine trips.

Atlant 15:25, 5 May 2006 (UTC)

Yeah you can get RCD outlets in the UK too but unlike your american ones they only protect thier own outlets. Also they are pretty expensive. Plugwash 17:32, 5 May 2006 (UTC)

Here, the going price is about US$12-$13, and they're frequently put "on sale" at $10 or so.

Atlant 19:17, 5 May 2006 (UTC)

[edit] RCD's and MEN connections

What are the implications of using RCD protection where MEN connections are present?

What are MEN conns? Is it the same as PME Protective_multiple_earthing (protective multiple earthing in UK?)--Light current 08:36, 5 May 2006 (UTC)

Assuming you mean a neutral thats tied to real earth at multiple points and/or used as a protective conductor as well as a neutral there are two reasons why a RCD shouldn't be used in such a core.

Paralell paths bypassing the RCD will cause it to trip when it shouldn't
Isolating a combined neutral and earth core is a BAD idea.

However its perfectly fine to feed a RCD from such a system provided neutral and earth remain seperate after the RCD and the only path from the outgoing neutral to the transformer neutral is to go through the RCD. Plugwash 09:09, 5 May 2006 (UTC)

[edit] alternative names for the device

at this forum thread lots of different names for RCDs in English have been listed. TERdON 22:36, 8 May 2006 (UTC)

[edit] Category: Electrical safety

I added this article to the category Electrical safety because I belive people looking for information on GFCI/RCDs are apt to look in that category.Gerry Ashton 14:34, 25 May 2006 (UTC)

[edit] Surge protector into RCD

Forgive my ignorance, but what's the typical result of plugging a surge protector into a RCD? Will both still function correctly? Nullbit 01:02, 25 September 2006 (UTC)

Yes it will Blaab

Thanks Nullbit

[edit] RCDs in Australia

The article doesn't seem to mention anything about Australia's regulations, so the short version:

RCDs must trip at no more that 30mA within 20ms (this is because anything over 35mA is likely to kill a person if it goes through their heart). RCDs must be installed for any new circuits added to a domestic situation (this includes lighting circuits). The only exception to this is where it would be deemed to add greater risk (such as emergency lighting or smoke alarms.) Also, if there is only one appliance wired into a circuit, it does not require an RCD (eg - an oven). It is common practice for electricians to run a dedicated circuit for older fridges that do not pose a risk. Also, RCD's are usually refered to as Safety Switches by the general public.

Hope that added to the info. Blaab 10:34, 28 September 2006 (UTC)

[edit] Cups of Rice?

These values were set by tests at Underwriters Laboratories during which volunteers holding cups of rice were subjected to shocks of known amperage and voltage.

What's the purpose of the rice? Is this possibly a joke?198.99.123.63 22:50, 29 November 2006 (UTC)

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