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#146279 - 11/23/06 10:44 AM GFCI vs. RCD
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Another thread, about portable generators, has me a bit confused.

I was under the impression that an "RCD" was the same as our "GFCI" (here in the USA).

A GFCI, or "ground fault circuit interrupter," is a device that -in simple terms- measures the current going out through the 'hot' wire and also measures the current combing back in the neutral. If the two figures are not very nearly the same (within 5ma), the device shuts off the power.
So, in the generator question, A shock would result in some of the current not returning to the generator, which would trip the GFCI.

I thought this was what the RCD did, but some have said that the RCD would not operate in this situation.

If the two are the same ... ho can it fail to operate?

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#146280 - 11/23/06 01:20 PM Re: GFCI vs. RCD
gideonr Offline
Member

Registered: 12/18/04
Posts: 152
Loc: Edinburgh, Scotland, UK
A GFCI is nearly equivalent to an RCD, but a generator is a special case, it might not have been grounded. Current must have a return path, which is usually ground for mains systems which would trip the GFCI/RCD, but with generators that aren't grounded the only return path is back to the generator, which if it goes around the GFCI/RCD and not by the neutral will trip the GFCI/RCD. If the shock current returns by the neutral then neither will trip.

Gideon.

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#146281 - 11/23/06 01:57 PM Re: GFCI vs. RCD
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
I agree that a fault that returns via the neutral will be seen as 'just another load.' A GFCI will not se that as a fault.

Yes, I think that current can flow into other places, away from the generator. After all, is not the wire "hot" to many things, besides the neutral that goes back to the generator?

If so, would not such current leakage be detected by the RCD?

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#146282 - 11/23/06 02:10 PM Re: GFCI vs. RCD
gideonr Offline
Member

Registered: 12/18/04
Posts: 152
Loc: Edinburgh, Scotland, UK
RCDs are usually 30mA trip, though occasionally 100mA trip are used.

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#146283 - 11/23/06 02:26 PM Re: GFCI vs. RCD
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
Reno I think you are right. Setting myself up to be shot down in flames, but, if the genset replaces the mains supply and is wired up correctly, RCDs or GFCIs should react the same way, surely? The device[s] should have no way of sensing any difference.

Alan
_________________________
Wood work but can't!

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#146284 - 11/23/06 02:31 PM Re: GFCI vs. RCD
Wolfgang Offline
Member

Registered: 09/25/05
Posts: 154
Loc: the very West of Germany
Two things mixed up here:

The main difference between the typical RCD and a GFCI is that the first is typically passive. It doesn't contain much electronic stuff and would work at 20 VAC as well as at 230V as long as more current is passing than allowed and written on it. On the other hand this principle limits trip minimum to 30mA for 3phase and 10 for 1phase devices.

The generator issue is something else. The very small gennys often have isolated conductors. There is no "hot wire" measured against the ground. It can be compared with "separating transformers" which we use here in Germany f.i. in tv repair shops. This issue is limited by a maximum power of some kW and a maximum length of the isolated mini-"grid". Otherwise a capacitive coupling (correct word?) could produce dangerous voltages against ground also with an isolated system. These limits are well defined in the IEC regs as far as I remember.

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#146285 - 11/24/06 03:25 AM Re: GFCI vs. RCD
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
I was under the impression that an "RCD" was the same as our "GFCI"

Effectively, yes, although as already stated our general-purpose RCDs are usually 30mA or 100mA trip rather than the 4 to 6mA trip of the American GFCI.

Current practice is to use a 30mA RCD where necessary (e.g. for receptacles used to power equipment outdoors). 100mA RCDs are still available and commonly used where RCDs have to be cascaded. A typical example would be a system where the main grounding of the installation is just to an earth rod rather than to the supply. The 100mA RCD would then be a delayed-trip type to provide protection to the entire system, with a 30mA RCD then used to provide the higher level of protection as needed.

10mA RCDs are also available, but rare.

 Quote:
if the genset replaces the mains supply and is wired up correctly, RCDs or GFCIs should react the same way, surely?

The key point is "wired up correctly." It's all going to depend on if and how the system is grounded.

Scenario #1. Genset frame is grounded by way of a rod, and one side of the 240V output is also bonded directly to the frame/ground. In this case the source will behave exactly like a normal (TN) supply, and the RCD will "see" any fault which exceeds its trip level.

Scenario #2. The 240V winding is not bonded at the genset (as is common on many small generators sold here), then somebody installs a bond to ground on the load side of the RCD to reference the system to earth. The RCD is now useless, since it cannot detect an imbalance in the installation's line/neutral current.

Scenario #3. The 240V output is completely floating, with no bonds anywhere. Again, the RCD would be ineffective. A bolted ground fault on either side of the installation would -- effectively -- just turn the system into scenario #2 above.

 Quote:
The main difference between the typical RCD and a GFCI is that the first is typically passive. It doesn't contain much electronic stuff and would work at 20 VAC as well as at 230V as long as more current is passing than allowed and written on it.

I'm not sure about the RCDs usually available in Germany, but in the U.K. there are plenty of active RCDs which incorporate an amplifier to increase sensitivity to the required level.

 Quote:
It can be compared with "separating transformers" which we use here in Germany f.i. in tv repair shops. This issue is limited by a maximum power of some kW and a maximum length of the isolated mini-"grid". Otherwise a capacitive coupling (correct word?) could produce dangerous voltages against ground also with an isolated system.


Capacitive is the right word.

Your "separating transformer" (direct translation from German?) is what we would refer to as an isolating transformer. Of course, this is exactly the principle used in our bathroom shaver outlets in Britain: The transformer is located right at the outlet so that the shaver supply is completely floating, and there is not enough wiring between the transformer secondary and the shaver for capacitive effects to become significant.



[This message has been edited by pauluk (edited 11-24-2006).]

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#146286 - 11/27/06 04:40 PM Re: GFCI vs. RCD
aussie240 Offline
Member

Registered: 07/06/05
Posts: 222
Loc: Blue Mountains, NSW, Australia
 Quote:
this is exactly the principle used in our bathroom shaver outlets in Britain

Paul,
Is it compulsory to have isolated sockets in UK bathrooms? So there's no 'proper' socket to run a portable heater then?
Here, we just have the normal non isolated 240V 10A outlet, although I did replace mine with the RCD type of GPO.

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#146287 - 11/28/06 01:56 AM Re: GFCI vs. RCD
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
In accordance with AS/NZS 3000:2000
All habitable areas require RCD protection, with a 30mA RCD, within a domestic installation.
This includes lighting circuits and socket-outlet circuits under 32A.
It does not include Air Conditioning circuits or circuits dedicated to alarms or TV amplifiers.
Refrigerator circuits are also exempted, where these are dedicated circuits.
Heaters in Bathrooms that afford Double Insulation, are also exempt, where they are supplied from thier own circuit.

[This message has been edited by Trumpy (edited 11-28-2006).]
_________________________
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#146288 - 11/28/06 03:41 AM Re: GFCI vs. RCD
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
Is it compulsory to have isolated sockets in UK bathrooms?

Yes, the only socket permitted in a bathroom under the IEE Regs. is a xfmr-isolated shaver type. It's been that way since at least 1955, if not longer. Of course, as I've pointed out before there has never been any compulsion to follow the Regs. in residential wiring, so it's not compulsory in the legal sense, and although regular sockets in bathrooms are rare, I've seen a few.

However......

This seems all set to change with the 17th edition of the Regs. due out in 2008. If all the proposed changes go through, then it will be permitted to install a regular outlet, subject to distance requirements and it being RCD-protected.

See IEE/BS7671, 17th edition



[This message has been edited by pauluk (edited 11-28-2006).]

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