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Discuss Why isnt the RCD tripping? in the Electrical Forum area at ElectriciansForums.net

happyhippydad

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I have just completed an EICR.
The install is TT, Ze is 308 ohms.
Zs on all circuits is <1.5 ohm due to bonding.
There is an up front 30mA RCD which trips with the push button but does not trip when tested either at a socket, light or actually in the RCD itself.
I know the RCD is faulty as I had a spare RCBO which I used and tripped as it should.

The lighting circuit had N-E IR readings of 0.27ohms. All other circuits were >2M ohms.

My question is why did the RCBO not trip instantly when I powered up this circuit through the RCBO?
 
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telectrix

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N-E fault. do you mean 0.27 ohms or 0.27 M ohms? either way a N-E fault can stop a RCD from tripping.
 

happyhippydad

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My initial post may sound a bit confusing as I'm tired!

Put simply, the IR test N-E is 0.27 ohms not M ohms and the RCBO does not trip when the circuit is powered up. It trips as it should do when tested.

Why does it not trip instantly when the circuit is energised with a N-E fault of 0.27ohms?
 

telectrix

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the N-E fault is inhibiting the RCD from tripping. the theory escapes me, but believe me, it's a fact.
 

Paignton pete

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My kewteck 64 does not trip an rcd on a TT system if the Ze is too high. If it’s over 300 - 400ohms.

This has happened a couple of times.

If I put an additional rod in lowering the Ze it works fine.

Don’t know why.
 

Risteard

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My kewteck 64 does not trip an rcd on a TT system if the Ze is too high. If it’s over 300 - 400ohms.

This has happened a couple of times.

If I put an additional rod in lowering the Ze it works fine.

Don’t know why.
Inhibition of excessive touch voltage.
 
The neutral return even though there is a fault to earth which sounds like a solid connection between the two may not necessarily decide to follow the earth path. It just keeps to the neutral because it is possibly the path of least resistance being a TT system hence no imbalance.
 
D

Deleted member 105166

If you clamp the lighting circuit L+N, what leakage value do you measure?
 

Des 56

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Your relatively high 270 thousand ohm resistance of neutral to earth (combined with a high resistance earth return) is sufficiently high enough that there is not enough leakage to earth to imbalance the RCBO in the normal working conditions
 

happyhippydad

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My kewteck 64 does not trip an rcd on a TT system if the Ze is too high. If it’s over 300 - 400ohms.

This has happened a couple of times.

If I put an additional rod in lowering the Ze it works fine.

Don’t know why.
Yes, but the zs is <1.5ohms.
 

happyhippydad

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Your relatively high 270 thousand ohm resistance of neutral to earth (combined with a high resistance earth return) is sufficiently high enough that there is not enough leakage to earth to imbalance the RCBO in the normal working conditions
It's 0.27 ohms Des, not 0.27M
 

davesparks

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There is an up front 30mA RCD which trips with the push button but does not trip when tested either at a socket, light or actually in the RCD itself.
I know the RCD is faulty as I had a spare RCBO which I used and tripped as it should.
When you tested at the RCD did you disconnect the outgoing conductors or disconnect L and N of every circuit?

How does the tripping of your spare RCBO prove that this RCD is faulty? Surely all that proves is that you have a working RCBO?
Testing the RCD with all outgoing conductors disconnected proves whether or not the RCD is working.
 
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