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bigal

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Hello everyone I'm newly registered and a bit green in finding my way around this site, but i'll do my best.
Here goes with my phenomenon.
Been working on a TT system, Just finished rewire of a house to 17th. Installation required a supply to an out building. Here my problem started.
I wanted to protect the cable going out of the house on a 30ma rcd (it was necessary). I Installed a garage board in the out building that also had a 30ma rcd as a main switch, so to avoid nuisance tripping I disconnected the outgoing cpc (the one from the main board in the house) at the garage board end and added another stake to give it it's own earth supply.
All seemed ok until I tested the rcd at the garage board end. It tests fine on x1 but when I test on x5 it trips the rcd at the house. This should not be possible because the outbuilding supply has not got a direct path back to the house because it is disconnected inside the garage board and relies solely on the nearby stake. earth electrode reading at the house (main earth was 20 ohm (loop test) and reading at outbuilding end 18.5 ohm. Any suggestions?
NICEIC technical say it's not possible. However, I've taken everything apart and checked for cross connections etc and put it all back together again. Still the same happens.
Help please. Bigal.
 
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M

mdavismccoy

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  • #2
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B

bigal

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  • #3
Thanks.
But, the cpc is diconnected at the out biulding end.
Th main board (house end) is protected by 100ma time delay but the next sub rcd within that board is 30 ma, as required and to my knowledge you cant get a time delay type 30 ma. I know the rod reading are good but are they possibly too good and causing a loop path direct to the house board?
Cheers
Bigal
 
S

Shakey

  • Thread Starter Thread Starter
  • #4
Bigal, its not causing a loop path

Think of how an RCD works (yes, I know you know!)

You are creating an imbalance between phase an neutral, which is what the RCD is there to detect.

The house RCD wont care where that imbalance is (because its downstream of it), it just sees an imbalance, the fact that the fault current is flowing down a seperate earth path (the garage spike) is irrelevant to the RCD.

Its absolutely pot luck which RCD will pop. I have a number of installations where I have a main 30mA, and a sub 30mA on each CU (they are training bays)

On an RCD test on any of the CU's certain ones will trip the CU RCD, some will trip the main RCD, some will trip both, and some will alternate which one they trip according to wether its tested on the positive or negative half cycle.

Why do you feel the need for a seperate RCD at the garage end? The circuit is adequately protected by the house RCD surely?
 
B

bigal

  • Thread Starter Thread Starter
  • #5
Shakey
Thanks
I could be being a muppet but I thought that if the cpc was disconnected at the garage end it would not be seen by the rcd at the house, even though it is on a TT system.
The reason for the rcd at the garage end is purely to save nuisance tripping. Who wants the house to go down by a fault created in the garage?
Cheers
Big al
 

Des 56

-
Arms
Esteemed
The best set up for a tt is the 100m/amp s type as the main switch with the sockets and whatever else on a 30 m/amp You could then run your outbuilding off the 100 m/amp and 30 m/amp rcd in the outbuilding This should stop your tripping probs at the house
Having a 30m/amp downstream of another 30m/amp is not a good idea
If you have one 30m/amp rcd covering the whole of the installation then there is no discrimination between lights and sockets and is frowned upon by many
I would say do not remove earths in a tt The extra rod in the outbuilding will improve the system at the least
 
B

bigal

  • Thread Starter Thread Starter
  • #7
Ok guys I think I need to reiterate.
Supply line is as such:
100ma s type as a main switch >30 ma x 2 as sub sections of the main board feeding various circuits in the house and on one of these rcd's is the supply cable to the garage>in the garage is a 30 ma garage board feeding lights and power. within this board I have isolated the cpc from the house (not connected it) the earth for this garage board is via its own stake.
The rcd test at the garage end should NOT see the rcd at the house because of this arrangement . So why does it trip the house 30 ma but only on x 5.
Big al
 

Des 56

-
Arms
Esteemed
The rcd trips when the phase and neutral are imbalanced
You are causing an imbalance in the supply entering the house irrelevant of where the earth path is being used
Phase to earth in the outbuilding via the local rod still causes an imbalance between the supply phase and neutral because the return path to the neutral carries less current than the outgoing phase because some or all of that return current is now going to earth
The rcds allthough 30m/amp still may have varying trip charachteristics
When you test x1 in the outbuilding it may operate that rcd before the household rcd can operate and vise versa on the x5 test
Still not a good idea to have one 30m/amp downstream of the other It does no help just causes these type of confusions
 
P

PC Electrics

  • Thread Starter Thread Starter
  • #9
Des is quite right.

You need to remove one of the RCDs - you have no discrimination.

Those NICe chaps should have spotted this flaw a mile off. Join ELECSA!!

Also bone up on how an RCD works
 
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B

bigal

  • Thread Starter Thread Starter
  • #10
Feel like I'm being told off now. However in my defence this was never a safety issue, just a nuisance problem. I understand the principles of the rcd, but thought that by creating an earth path closer to my control, especially one that had a slightly better Ra the this would create the tripping preference.
If I remove one of the rcd's (it would have to be the one in the garage to comply) surely the annoyance from the house when a rodent chews through a cable overnight and plunges 1/2 the house into darknes and looses all the savings on little Johnny's X box game I won't be flavour of the month. Maybe this is a dig against the industry that produces limited rcds and regulatory boards that demand higher and higher control standards.
Big al
 
S

Shakey

  • Thread Starter Thread Starter
  • #11
Bigal, Sorry to hear you are being told off

maybe if you werent such a naughty boy.....:p

Think of the current flow through the circuit, exactly the same current is floing though the first RCD as the second, so the fact that you have a better Ra at the garage just produces a better fault to earth current AT BOTH rcd's!

And as you have proved (and as I have explained) it is abosilutely pot luck as to which RCD pops, wether you are testing,OR if roddy the rodent has a nibble at the garage end

Ergo, your garage RCD is serving no useful purpose whatsoever

So, bin the second RCD, buy little Johnny a memory card for his Xbox, put some rat killer down in the garage, and wonder why the NICEIC often rub toilet paper on their elbows:p:p:p
 
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PAUL M

  • Thread Starter Thread Starter
  • #12
why dont you put a 10mA rcd in the garage then you have got discrimination.
 
E

EasyFox

  • Thread Starter Thread Starter
  • #13
Hello everyone I'm newly registered and a bit green in finding my way around this site, but i'll do my best.
Here goes with my phenomenon.
Been working on a TT system, Just finished rewire of a house to 17th. Installation required a supply to an out building. Here my problem started.
I wanted to protect the cable going out of the house on a 30ma rcd (it was necessary). I Installed a garage board in the out building that also had a 30ma rcd as a main switch, so to avoid nuisance tripping I disconnected the outgoing cpc (the one from the main board in the house) at the garage board end and added another stake to give it it's own earth supply.
All seemed ok until I tested the rcd at the garage board end. It tests fine on x1 but when I test on x5 it trips the rcd at the house. This should not be possible because the outbuilding supply has not got a direct path back to the house because it is disconnected inside the garage board and relies solely on the nearby stake. earth electrode reading at the house (main earth was 20 ohm (loop test) and reading at outbuilding end 18.5 ohm. Any suggestions?
NICEIC technical say it's not possible. However, I've taken everything apart and checked for cross connections etc and put it all back together again. Still the same happens.
Help please. Bigal.
Split the tails & use a small sub-board with 100A main switch & an 30mA rcbo for the submain to garage then bin the rcd in the garage. That'll get you over the house being plunged into darkness & the xbox that Johnny is on.
Or
In future use a dual rcd 17th edition cu that has a main switch with non rcd ways & use an rcbo for the submain.

Shant bother with why the rcd's trip & discrimination stuff as Shakey & others have been there.

Now I have to do these quotes
 
P

PC Electrics

  • Thread Starter Thread Starter
  • #14
Paul M

A 10mA RCD in the garage WILL NOT give RCD discrimination.

Didn't mean to 'tell you off' Bigal - just pointing you in the right direction succinctly - I only type with two fingers so it would take me ages to explain the issue whereas you could find tons of useful resource quit quickly using Ask or Google.
 
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