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Topic Title: Real problem guys really need some help?
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Created On: 18 December 2015 12:38 PM
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 10 January 2016 02:56 PM
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alancapon

Posts: 6881
Joined: 27 December 2005

Originally posted by: GLMelectrical
. . . I distinctly remember seeing HAGER advising their RCBOs shouldn't be used on TN-C-S earthing arrangements . . .

It would be useful if you can provide a hager reference to back up that statement.

Regards,

Alan.
 10 January 2016 03:03 PM
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GLMelectrical

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trust me...I went looking last nite.....and couldn`t find any.....but I wouldn`t have posted it up in the first place if I hadn`t seen it someware....but it was a few years back now Al...
 10 January 2016 03:05 PM
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GLMelectrical

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it may not have been HAGER....but I`m sure I saw it....one of the major manufacturers...
 10 January 2016 03:29 PM
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leckie

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RCBO's not suitable for TNCS? Well that's not really likely is it Glenn? Think about it, you are saying SP RCBO's should not be send on TT and that a certain major manufacturer says their RCBO's are not to be used on TN-C-S systems. So where exactly would this manufacturer expect you to install their product? Bear in mind that the vast majority on TN-S systems are in fact essentially the same as TNCS as they are jointed as TNCS externally to the installation, in the street or wherever. So the SP RCB's of this major manufacturer can basically not be used anywhere! I think you are mistaken, and can absolutely assure you that Hager do not say that, or Schneider, Control Gear, Contactum, Havels, Crabtree, Eaton, Wylex, etc.

Regarding the testing of an RCD output terminals even at 250V, what does that indicate? What do you mean by "flat" between L an N?
 10 January 2016 04:50 PM
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GLMelectrical

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I mean IR was virtually 0 MEG between L&N..

it was this one time where I went to a callout....

A BG engineer had failed the cooker...saying it was the cooker that was causing an RCD to keep letting go....they had replaced the cooker...with no improvement..

I went in and found the RCD to be low IR between L&N secondry side...

Obviously I put a new RCD in....with no further problems..
 10 January 2016 08:40 PM
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AJJewsbury

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TT of course is a no no....( unless DP of course)

What's wrong with a SP RCBO on TT? (unless there's a requirement for individual isolation of each circuit and no desire to supply another device to serve that purpose, or for discrimination with upstream RCDs). ADS only requires to the supply to the line conductor to be interrupted (reg 411.3.2.1), not full isolation.
- Andy.
 10 January 2016 09:43 PM
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leckie

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I think that is true Andy, but we all have out own ways of doing things.

I haven't fitted an AMD3 board with a TT system yet, but when I do I will be fitting an upfront 100mA S type incomer and DP rcbo's as required for additional protections. I want the maximum chance of discrimination between the devices and for this to include N to E faults. This is how I have done it previously with insulated boards, even though if all RCBO's I can see an argument for just a main switch incomer in that case. But if I'm going into a metal box I want fault protection at the earliest possible opportunity, so I would want the RCD incomer.

However fitting SP RCBO's in a TT installation is not a no no, although some installations might, as you say, require DP isolation for example filling station pump circuits.

Edited: 10 January 2016 at 11:39 PM by leckie
 11 January 2016 12:16 AM
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alancapon

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Joined: 27 December 2005

Originally posted by: GLMelectrical
I mean IR was virtually 0 MEG between L&N . . .

. . . I went in and found the RCD to be low IR between L&N secondry side...

Obviously I put a new RCD in....with no further problems..

I assume that your IR test was of the RCD on its "load" terminals. What you are likely to have measured is the dc resistance of the electronics inside the device. This does not show that the RCD is faulty - it is simply the way it is made. The only method of testing an RCD is with a purpose made RCD tester on either a "ramp" or "timing" test.

Regards,

Alan.
 11 January 2016 08:57 AM
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mapj1

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And to add further to Alan's comments, note that testing is only reliable if the L-N and L-E voltages are close to nominal - the RCD may not work well if the line voltage is low or noisy. Luckily the modern testers do warn you if it measures a low L-N or a large N-E offset.
And, as Goldenboy discovered the hard way, when faced with a circuit with a misbehaving RCD, the tool you really want to have in the box for measuring your safety margin is the modern extra sensitive milli-amps clamp on meter of the kind you can get around the N and L wires as they leave the RCD terminals. A moderately cheap one will do , and can pay for itself the first or second time it saves you a morning of disconnecting things blindly to see if that improves the problem or not.

-------------------------
regards Mike
 11 January 2016 03:17 PM
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GLMelectrical

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But iv had them where they,v Been clear...ones that have been faulty have been low between load terminals...hence why I suggested it in the first place...
 11 January 2016 03:20 PM
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GLMelectrical

Posts: 71
Joined: 05 January 2016

Agreed mike about applying leakage clamps on tails....and not on main earthing conductors
IET » Wiring and the regulations » Real problem guys really need some help?

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