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Topic Title: Up front RCD
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Created On: 15 August 2014 05:33 PM
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 15 August 2014 05:33 PM
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mickjeff

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Is an up front 100mA RCD(not S type) which is installed before 30mA dual rcd consumer unit against current BS 7671 regulations? The upfront trips before 30mA at 150mA taking out the whole consumer unit which in my opinion does not meet 314.1
 15 August 2014 05:47 PM
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sparkingchip

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Would you like to review your test results? I read that as the 100mA RCD trips at 150mA, but the 30mA RCDs haven't tripped?

Andy
 15 August 2014 05:53 PM
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sparkingchip

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Are you saying you did a 5X test on each of the 30mA RCDs and the 100mA rcd went first without the either 30mA RCD tripping first?

If so what was the tripping time for each of the three RCDs at 150mA?

Andy
 15 August 2014 05:59 PM
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mickjeff

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Originally posted by: sparkingchip

Would you like to review your test results? I read that as the 100mA RCD trips at 150mA, but the 30mA RCDs haven't tripped?



Andy

Yes that is correct Andy not sure why you are asking to review test results.
Whilst testing from a circuit from one of the 30mA RCD the up front RCD tripped first at 150mA disconnecting the whole installation does this breach 314.1?
 15 August 2014 06:18 PM
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Legh

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Originally posted by: mickjeff

Originally posted by: sparkingchip

Would you like to review your test results? I read that as the 100mA RCD trips at 150mA, but the 30mA RCDs haven't tripped?

Andy


Yes that is correct Andy not sure why you are asking to review test results.

Whilst testing from a circuit from one of the 30mA RCD the up front RCD tripped first at 150mA disconnecting the whole installation does this breach 314.1?


It depends what you mean by breach. Non-conformance to a regulation included in a statutory document might be regarded as a breach but choosing not to adhere to a guidance note is probably more like a deviation.

Consider the use of two G type 30mA series connected RCDs that you might find on a caravan site or boatyard, they would certainly be a deviation from 314, but hardly a breach.

Legh

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 15 August 2014 06:23 PM
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sparkingchip

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I'm just asking you to confirm that all three RCD's are within their specification, if the 30mA RCD is sluggish then the 100mA is doing a good job as it is tripping within it specification as it should do.

I can't see it is against the regulations and swapping it out for a 100mA s-type RCD or a non-RCD main switch may be a preferred choice depending on the particular circumstances of the installation, but technically I think it can be left as is.

What do you think should be done?
 15 August 2014 06:25 PM
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mickjeff

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It depends what you mean by breach. Non-conformance to a regulation included in a statutory document might be regarded as a breach but choosing not to adhere to a guidance note is probably more like a deviation.

OK let me rephrase my question is it OK to use an up front 100mA RCD (non time delayed) in a new installation?
 15 August 2014 06:26 PM
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weirdbeard

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Originally posted by: mickjeff

taking out the whole consumer unit which in my opinion does not meet 314.1



Hi MJ, was the owner about to give anyfeedback on the situation?
 15 August 2014 06:29 PM
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mickjeff

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Originally posted by: sparkingchip

I'm just asking you to confirm that all three RCD's are within their specification, if the 30mA RCD is sluggish then the 100mA is doing a good job as it is tripping within it specification as it should do.



I can't see it is against the regulations and swapping it out for a 100mA s-type RCD or a non-RCD main switch may be a preferred choice depending on the particular circumstances of the installation, but technically I think it can be left as is.



What do you think should be done?


I think it should be replaced with an isolation switch, I think whole point of using a split consumer as 314.1 guides us so that the whole installation is not isolated due to a fault.
 15 August 2014 06:39 PM
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mickjeff

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Originally posted by: weirdbeard

Originally posted by: mickjeff



taking out the whole consumer unit which in my opinion does not meet 314.1






Hi MJ, was the owner about to give anyfeedback on the situation?


Hi WB,
Not sure what you mean the owner of what? the installation? I doubt they would understand but would be unhappy if a fault on one part of the installation disabled everything.
 15 August 2014 06:41 PM
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sparkingchip

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What is the reasoning behind having a 100mA RCD main switch upfront of the installation

Is it a TT installation where the 100mA is upfront of the double insulated singles tails, if so how long are the tails and do they have reinforced insulation being run in plastic trunking to the appropriate British Standard or is it a SWA distribution circuit, is there any need to RCD protect them?

Is the 100mA the main switch within the consumer unit with less than three metres of double insulated singles tails and PME or TNS?

Andy
 15 August 2014 06:54 PM
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mickjeff

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Originally posted by: sparkingchip

What is the reasoning behind having a 100mA RCD main switch upfront of the installation



Is it a TT installation where the 100mA is upfront of the double insulated singles tails, if so how long are the tails and do they have reinforced insulation being run in plastic trunking to the appropriate British Standard or is it a SWA distribution circuit, is there any need to RCD protect them?

Is the 100mA the main switch within the consumer unit with less than three metres of double insulated singles tails and PME or TNS?
Andy


There isn't any point in having a 100mA up front RCD of a new installation. My reason for asking this question is I came across an installation today where a new CU has been installed using existing wiring and also leaving the up front 100mA RCD in place. I personally would have removed it and replaced it with a double pole isolation switch. All I am trying to gain here is some understanding whether it would be OK to leave the up front RCD or remove it for compliance with current regulations.

The tails are less than 3m about 1m if that on a TN-C-S install.
 15 August 2014 07:07 PM
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OMS

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As it stands, I'd suggest that it's not really compliant with BS 7671 in terms of avoiding hazard or minimising inconvenience.

You could mitigate by using a 100mA Type S (but a bit pointless) or swapping for a typical KMF unit if the consumer unit has no main switch.

At worst, just remove it and tail straight from the meter to the consumer unit (assuming the con unit has a single main switch)

Worth checking those 30mA units if you do remove it - presumably you haven't actually been able to prove thier operation as yet

Regards

OMS

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 15 August 2014 07:13 PM
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mickjeff

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Thank you OMS that is all I needed, in my own mind I thought leaving the up front RCD was not compliant with current regs but judging by the rest of the install it is probably down to laziness old green sleeving on cpc etc.

No haven't been able to test 30mA RCD's as yet client was going on holiday will test them after removing 100mA up front RCD.
 15 August 2014 07:18 PM
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sparkingchip

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I'd go down the route of swapping it for a non-RCD main switch to save meter seal issues, but that in turn raises other issues
 15 August 2014 07:21 PM
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weirdbeard

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Originally posted by: sparkingchip

What is the reasoning behind having a 100mA RCD main switch upfront of the installation



Sometimes a good condition tested upfront device makes a handy isolator to change the DB...... By adding a dual 30mA DB to the existing upfront rcd you don't alter the existing fire protection or level of nuisance trips likely to happen in the future, you are simply adding extra 30mA rcd protection to random circuits of the house so that if someones had a zap doing some diy on the downstairs lights, at least the TV in the front room will still work !


 15 August 2014 07:23 PM
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weirdbeard

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Originally posted by: mickjeff



I think it should be replaced with an isolation switch,


Changing to an S-type might improve the existing level of inconvenience suffered, whilst retaining the existing additional layer of safety - give or take a few milliseconds, and also the mark up on s-type rcds is better than isolators
 15 August 2014 07:23 PM
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mickjeff

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Originally posted by: sparkingchip

I'd go down the route of swapping it for a non-RCD main switch to save meter seal issues, but that in turn raises other issues


For some reason seals are already missing so not an issue up to now.
 15 August 2014 07:27 PM
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GeoffBlackwell

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We have not yet been told why the front end RCD was fitted in the first place.

Is this installation connected to a TT system?

Is this a new installation, and if not how old is it?

A front end RCD might be required in some circumstances - even if all outgoing ways are protected by other RCDs.

So a complete description of the set up is needed before any definitive answer can be given.

Regards

Geoff Blackwell
 15 August 2014 07:32 PM
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sparkingchip

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A few posts back mickjeff slipped in: "The tails are less than 3m about 1m if that on a TN-C-S install."

It sounds like he is capable of making a informed decision, but looking to justify his actions if asked why he altered the previous electricians work.

Andy
IET » Wiring and the regulations » Up front RCD

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