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Re: UK cooker/RCD problem #143859 09/23/05 05:02 PM
Joined: Sep 2004
Posts: 93
J
jooles Offline
Member
The "whole-house" thing may be more dangerous than none at all for another reason.

Take a scenario where at bedtime something in the house (someone mentioned old fridges? could be anything actually) causes a nuisance trip. There was no actual electrical danger to anybody, but it has now plunged the whole house into darkness.

In the course of making your way to the meter cupboard to reset the pesky thing, you trip over the cat on the stairs, fall to the bottom, and sustain a broken leg, or worse.

The dangers of having fixed lighting circuits protected in this way seem way out of proportion to the (very small) chance that someone will get a fatal shock off a ceiling fitting.

Perhaps more debatably, nor would I be very keen on the idea of a deep freezer full of food getting knocked offline by a nuisance trip while we went away for two weeks in the middle of summer. I wonder what the insurance broker would have to say about the claim for replacement. They will certainly pay the claim if the freezer itself develops a fault, but when we talk about the electricity being turned off for some other reason, there's too much wriggle room there for my ease of mind.

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Re: UK cooker/RCD problem #143860 09/23/05 08:05 PM
Joined: May 2004
Posts: 186
A
aland Offline
Member
Jooles, Good point thats why we have split load type boards, Lighting on None Rcd side or 100ma. Socket circuits and other vulnerable equipment on 30ma trip. If you do put a 30ma trip across the whole lot. You might leave yoursefe open to being sued in the event of a tumble.

Re: UK cooker/RCD problem #143861 09/24/05 07:20 AM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline
Member
Quote
Why we dont use rcbo's more redily I will never know


I think we're in something of a Catch-22 situation at the moment. People will balk at installing RCBOs until the prices come down on them, but the prices won't drop until they're more widely used.
Modern split-load boards help, but they're not without their own problems out here in rural TT-earthing land. By the time you factor in a 100mA time-delay RCD for the main on top of the 30mA RCD sub-main, it can start getting quite expensive.

There's always the option of converting to PME and installing a "normal" split load board, but that's likely to be just as expensive when much of the bonding then needs to be upgraded.

There are still a lot of houses around here on TT with an old voltage-operated ELCB.

Re: UK cooker/RCD problem #143862 09/25/05 05:07 AM
Joined: Jul 2002
Posts: 8,262
Trumpy Offline
Member
AB,
Quote
Checks with a multimeter reveal no obvious anomalies.

I'd get the thing meggered, this could show up a few things

Re: UK cooker/RCD problem #143863 09/25/05 06:31 AM
Joined: Aug 2001
Posts: 7,520
P
pauluk Offline
Member
Yep, a low-voltage ohmmeter test will not always reveal problems that a high-voltage test will show up.

I had an outdoor pond pump like that recently in which damp had gotten in via a crude splice. A low-voltage ohmmeter test showed resistance over 20 meg, yet putting it on a 500V megger revealed the problem instantly.

Re: UK cooker/RCD problem #143864 09/25/05 04:47 PM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
A cooker unused for a longer period of time!

I know internal recommendations of a well known manufacturer, that you are supposed to run and heat the equipment for some time in order to reduce leakage current.

In case of a TT installation with rcd tripping it is recommended to isolate the cooker/oven and to unmount (!) PE conductor in order to achieve drying. Better don't touch it in this phase.

It is not worth to meg the device (if it has not a low ohm failure) before half an hour of ordinary service.

This is not my personal idea!

Re: UK cooker/RCD problem #143865 09/26/05 06:41 AM
Joined: Oct 2004
Posts: 48
M
marcspages Offline
Member
Hi Guys,

Yip, heard of this one too (disconnect the Protective Earth so as to stop the ECLB/RCD/RCBO - take your pick!) from tripping. Once had a case where this was done and tester/operator was receiving "mild tingles" (i.e. shocked like hell!). It revealed the knobs were conductive!

Solution: Not to disconnect the Earth, but to rather have a non RCD line (or to bypass the RCD) feeding the cooker for the burn-in period but to monitor the leakage current to ascertain the burn-in was complete. If an element failed (shorted) during this time it would either pop the fuse in the cooker (some older cookers had fuses per circuit) and/or trip the main current breaker (usually at the same time the filament tried to escape through the element casing!).

I have continued this practice through all cases of earth leakage problems (especially cookers) by bypassing the RCD but monitoring the leakage current. When the leakage falls below the required mA, return the circuit to normal (reinstate the RCD) and leave behind one very happy customer (and no-one shocked in the process). You'd be surprised at the number of central heating pumps that need this treatment too.

M.

Re: UK cooker/RCD problem #143866 09/27/05 09:05 PM
Joined: Dec 2004
Posts: 354
K
kiwi Offline
Member
I haven't seen any advertising claiming rcd protection is the " be all and end all " solution to electric shock danger.

When I say all equipment should be rcd protected, I actually meant in the domestic situation. Obviously some equipment in industry will have its exclusions

Single RCD main switches never really took off in NZ. Most sparkies here will have a few in the domestic DB ( 2 lights & 2 power ) or use RCBOs. Eliminating the "plunged into darkness" scenario.

RCDs are just one more tool to minimise electric shock risk and shouldn't be ignored just because of a little nuisance tripping.

Re: UK cooker/RCD problem #143867 09/28/05 04:41 AM
Joined: Oct 2004
Posts: 48
M
marcspages Offline
Member
Kiwi,

I am fully with you that RCDs are in no way a protection against electric shock, that's what the protective Earth conductor is for.

I believe RCDs are mis-sold. I personally view an RCD as a means to detect latent faults i.e. where a current carrying conductor is attempting to return to its birth place (i.e. Earth/Ground). Usually this involves a high-ish resistance (such as water) and if it was not for the RCD the fault could escalate to a point where it becomes threatening (such as fire).

In short; The RCD is simply another component within the distribution system to protect the electrical installation and devices connected to it, and not as a means to prevent electric shock.

M.

Re: UK cooker/RCD problem #143868 09/28/05 05:36 AM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
In this discussion do not forget that in certain power grids a protection in case of an isolation fault can only be realized by means of an RCD because the loop impedance is to high. In typical TT-systems (France, Belgium for instance) earth leakage even in case of a full short to earth is that weak that a fuse will never trip.

That's why in France RCD's are usually integrated in the mains switch delivered by the PoCo.

So the RCD discussion is not the same for everybody.

[This message has been edited by Wolfgang (edited 09-28-2005).]

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