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Topic Title: RCD's....Does it matter if which way supply is connected.
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Created On: 06 February 2013 10:51 PM
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 06 February 2013 10:51 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Evening all,
Dont shout at me for asking this question but does it matter whether an RCD is supplied from the bottom or the top.
I dont think it matters as the internal is a resistor load so can go either way.
I just thought I would ask anyway.
Regards
Antric
 06 February 2013 11:06 PM
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primo

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I once connected a contactum RCD with supply at the bottom and when energised it started smoking and melted! Still notnsurenif this was because of a faulty RCD or whether it didn't like it the other way round! As a result of that episode I now try to always put the supply in the top of any make!
 06 February 2013 11:37 PM
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MickeyB

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The RCD will trip when the load is switched on.
 06 February 2013 11:42 PM
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ebee

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As far as I know I think the answer is some Yes some no.
I depends upon the manufacturer.
Ditto ref getting 1(Ph) & N transposed

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 February 2013 10:11 AM
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jcm256

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Have these RCDs gone out of favour, not many grant you but came across them in distribution boards from time to time. Has technology moved on or the neutral considered more reliable nowadays.

(Wikiepida describes it well)

A dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding live wire remains uninterrupted. If the tripping circuit needs power to work, it cannot operate. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with the energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the live wire is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.
 07 February 2013 07:09 PM
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dbullard

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Many many moons ago I had a site with 140 RCD's to test, after a while you get used to testing at certain part of the RCDi'e bottom on the outgoing side, then I had around 20 failures in a row, it transpired that someone had wired them the other way around and dummy here hadn't realised .

I contacted Hagar technical regarding this and their answer was

"It makes no difference to which the incoming or outgoing side are connected, and will not affect the operation or safety of the device"

After checking it would appear correct as the tripping times were within the BS guidelines etc, just confused me as I was in "autopilot" while testing

Hope it helps

Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 07 February 2013 09:20 PM
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antric2

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I know that RCD fused spurs have to be correct way round as they tend to give a type of BANG noise when powered up if supply and load are reversed.......or so I am told..honest !!

Thank you all for your responses.
Regards
Antric
 07 February 2013 10:40 PM
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stateit

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Some of 'em don't like it up 'em Capt. Mainwaring !!

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S George
http://www.sg-electrical.com
 08 February 2013 12:28 PM
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geoffsd

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As Hager have stated that it does not matter to which end the supply/load are connected -

Would you therefore conclude (logically, despite what anyone may say - as you have, in effect, reversed the internal wiring) that the polarity cannot matter either.

Notwithstanding complying with the MIs as some manufacturers mark their devices and some don't.
 08 February 2013 12:55 PM
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AJJewsbury

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Would you therefore conclude (logically, despite what anyone may say - as you have, in effect, reversed the internal wiring) that the polarity cannot matter either.

I often wondered if some devices are arranged so that N opens last and closes first, if they did then reversing load/supply wouldn't make a difference, but swapping L/N would. Certainly some RCBOs have a proper arc quenching contact on L but just a simple switch contact on N.
- Andy.
 08 February 2013 01:07 PM
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OMS

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I often wondered if some devices are arranged so that N opens last and closes first,


You would need to deliberately specify that, Andy - in the vast majority of cases, the requirement is that the poles break "at substantially the same time"

Typically, delayed neutral switching in large UPS supplied systems on changeover gear that keeps the N-E bond in place until the alternative source neutral is earthed. Closed transition systems normally

Regards

OMS

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Failure is always an option
 08 February 2013 01:17 PM
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ebee

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I did once have an installation where the DP switch for immersion heater tripped the RCD almost every time at switch off but never at switch on.
All seemed bone dry, no great amounts of muck inside the switch or backbox and all IR readings respectable.
I replaced the switch and problem disappeared.

My only conclusion was that one pole of the switch might be lagging slightly behind the other hence causing problem.

Is that possible?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 08 February 2013 02:02 PM
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OMS

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Entirely Ebee

OMS

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 08 February 2013 02:14 PM
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dbullard

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Had the same situation Ebee not only on Immersion heaters but also cooker panels, and some DP 2 gang sockets.


Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 08 February 2013 02:30 PM
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AJJewsbury

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Although you'd imagine that having the N end of the element at 230V for a short while shouldn't of itself be a problem unless the N-E insulation was poor.

Maybe there's a bit of arcing that's introducing some high frequency components into the current that then pass to the element's earthed sheath through capacitive coupling? Coupled with an open N, the entire element would act as a capacitor, rather than the effect declining towards the N end as the voltage difference declines. Just a theory.

- Andy.
 08 February 2013 03:35 PM
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ebee

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Well I`m reet glad about `cos everyone else I mentioned this scenario to said I was talking absolute bolloks.

Firstly those that said if it trips at switch off it would trip at switch on too , so I must be mistaken and second those that did believe me said it must be another reason .



-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 13 February 2013 03:20 PM
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AJJewsbury

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I often wondered if some devices are arranged so that N opens last and closes first, if they did then reversing load/supply wouldn't make a difference, but swapping L/N would.


I've just noticed that MK say for their switch disconnectors "Make first, break last on neutral" - http://www.mkelectric.com/Docu...%20Tech%20661-689.pdf

- Andy.
 13 February 2013 10:39 PM
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antric2

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Originally posted by: AJJewsbury

I often wondered if some devices are arranged so that N opens last and closes first, if they did then reversing load/supply wouldn't make a difference, but swapping L/N would.




I've just noticed that MK say for their switch disconnectors "Make first, break last on neutral" - ">http://www.mkelectric....ocu.....661-689.pdf



- Andy.


That is a good info sheet ,well found Andy
Yesterday, to add to the confusion, I unboxed an RCD made by ABB and there was no indication on it for L or N and no reference to the polarity in the paperwork so as already said, it all depends on the RCD and to follow the manufacturer guide lines,if any.
Regards
Antric
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