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100mA RCD Tripping

Postby charliewm » Thu Dec 17, 2009 6:59 pm

Hello All,

We have a situation in a barn conversion as follows: the supply is three phase, with one phase connected to a dual 17th Edition MK CU. At the intake position which is in an outbuilding 50 metres away, the supply comes in SWA to 100 Amp 100mA three phase RCD. THen to 100Amp switch fuse and then supplies both the barn and other separate outbuildings. The supply is all in SWA. The problem is that sometimes the 100mA RCD trips and not the 30mA RCD on the CU. We have done a Ramp test on the 100mA RCD and the results are Phase 1 80mA, phase 2 80mA, phase 3 70mA.
The supply to the main intake position is TNCS and the barn is TT.
My feeling is that as the supply is all mechanically protected (SWA), we could change the 100mA RCD for a Main Switch but we would still like to know what is the cause of the tripping. All insulation tests are good.

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Postby ericmark » Fri Dec 18, 2009 2:01 pm

I would think if a 100mm RCD is tripping then you need it.

Basics why does one use a 100ma RCD?
1) Earth loop impedance is not good enough to be able to blow the fuse without it. So what is the earth loop impedance reading?
2) Fire protection. Is there any flammable items keep in the buildings? Well you have already answered that it feeds a barn.
So only option is to replace with 300ma RCD if the ELI will permit or get rid of reason why it trips. I would think latter is really the way to go.

So again start at basics. What is the insulation resistance readings?

There are two options. Methodical and Intuitive. So either you slowly test each and every bit or you sit back and think of likely causes. Submersible pump for example.

The third is to use more RCD’s and split the work of each one. Since any RCD can trip with half it’s rated value we normally use 1/3 rule. 10ma x 3 = 30ma x 3 = 100ma x 3 = 300ma but an S type 100ma does not normally trip without reason.

Normally all down to cost. It is often cheaper to split the supply into circuits than to spend loads of time fault finding.

The Vigi range of RCBO’s from Schneider we found very good fitting into their Isobar boards these add on residual current devices allowed one to fit MCB and RCD 3 phase protection to each supply where as using other options one could not fit both MCB and RCD into same outlet.
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Postby moggy1968 » Tue Dec 22, 2009 11:01 pm

what type of RCD is it?
the problem you have here is to do with discrimination. If you use a time delay 100mA RCD then your 30mA ones will trip first. I did this recently on my mum and dads garage supply as their board is in the attic and I didn't want my dad trailing up 3 flights of stairs everytime the rcd tripped.
If you have 100Ma and 30 MA RCDs of the same type often they will both trip as the trip current will be more than 100mA. If the 30mA is a little more 'sticky' than the other then then the 100mA may trip first
You do need to identify where the fault is though if it keeps tripping. If you fit an main switch without RCD then you are treating the symptom, not the cause
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Postby charliewm » Wed Dec 23, 2009 5:58 pm


Thanks for both your replies. Situation has become clearer. There is a fault on a lighting circuit. The insulation readings are fine and my suspicions are a switchwire in the neutral block in a fitting causing a short circuit. That said, the property is to be let out (it has been converted into 5 bed house) and we have to sort out the discrimination problem. The 30mA RCDs on the CU both trip at 10mA as Ericmark suggested. I suspect that the 100mA RCD is there because of a high Loop reading and it is not part of our brief. The course is probably a time delay 100mA replacement RCD if the client agrees as they cost a lot. Again many thanks and Happy CHristmas.
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