RCD problem...nuisance tripping

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xr4x4

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hello,

long story so ill try and keep it short...

our mains supply goes into a stand alone 80amp 30ma RCD then into a industrial fuse box with a TT earth.

this box then feeds us with MCBS to sockets lights etc. there are also several supplys going out via SWA cable to a few out buildings.

the one in question a 50amp mcb that feeds another std CU via SWA to another building...pic below.

starbreaker.jpg


first question, is that a RCD in above pic? if so, why if there is a problem at that end with a supply from the crabtree CU, does it trip out the main RCD... IE us... everything goes dark...!!

is it because its a 100ma RCD and the first main one being 30ma trips first?

if this is the case, can the 100ma RCD in the crabtree be changed for a 30ma one stoping our nuisance tripping?

ie this... http://cgi.ebay.co.uk/STARBREAKER-R...UK_BOI_CircuitBreakers_RL?hash=item45f02f6654

i guess the above would need a main ISO too?

thanks in advance.
 
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I'd have thought the 1st RCD in from the street should be a 100ma for a TT system NOT 30 ma

Discrimination would then mean any sub boards should have the lower 30ma rating.

What's not clear is what bits other than the main feed is TT, does each sub board have it's own earth rod or do the feed out cables from the primary each extend the earth ?
 
You have to ask why is there a RCD it could be.
1) Because of high earth loop impedance (Could be any size according to calculations)

2) To protect for fire (normally 300ma)

3) To protect personnel (normally 30ma)

It would normally reduce by 1/3 each time so 300ma feeds 100ma with feeds 30ma which feeds 10ma. Also often you get delay as well (S type)

So the two RCD's you show do seem wrong way around but what does the first on protect?

If there are any cables buried in a wall or sockets under 20A (Except farms etc) then you need 30ma for that so can't just change. You may be able to split supply before the 30ma RCD but really this is not DIY work.

There is a big red book with all the regulations not easy to work out without being on site and doing a full risk assessment.
 
hmm... it is kinda a farm.

ill try and answer questions in order..

it deffo says 30ma RCD, this is at beginning of supply, so, cut out, meter, RCD, its a wylex unit, them from this into main CU.

all the earths AFAIK all come back via org source cables to main earth in main CU. ie the SWA cables. im pretty sure there is only one earth rod and thats out side here.

i thought all TT earth systems must be RCD protected.

the first RCD on the main incomer protects everything its connected to.
 
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There is no point having a 100mA RCD upstream with a 30mA RCD downstream.

Just think, if you get an earth fault of 100mA. Which RCD will trip first?

It could be either - it will be the one with the fastest action time.

What you should have at the incomer is a
100mA TIME DELAYED RCD (S-type)

At the other DBs there should be 30mA RCDs.

Now the 30mA will always trip first when you get your 100mA fault.
 
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Taylortwocities said:
There is no point having a 100mA RCD upstream with a 30mA RCD downstream.

Just think, if you get an earth fault of 100mA. Which RCD will trip first?

It could be either - it will be the one with the fastest action time.

What you should have at the incomer is a
100mA TIME DELAYED RCD (S-type)

At the other DBs there should be 30mA RCDs.

Now the 30mA will always trip first when you get your 100mA fault.
Click to expand...

hmm..

so it sounds like the main RCD needs upgrading, trouble is you cant (or i cant find) standalone RCDs any more and im not sure the main CU will take a internal RCD.

:confused:
 
the main CU is a Merlin gerin isobar 4 evo 18 way BTW.

can an internal RCD be installed in this?
 
Post a PIC of the CU. Lets have a look but it really needs a thorough review.
Much will depend on teh supply type and what is connected.

PS, Isn't this your landlord's responsibility?
 
Taylortwocities said:
Post a PIC of the CU. Lets have a look but it really needs a thorough review.
Much will depend on teh supply type and what is connected.

PS, Isn't this your landlord's responsibility?
Click to expand...

ill get a pic.

it is, but they are very busy, ill put in the home work and put what needs doing to them, also learn along the way.

thanks.
 
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