Why isnt the RCD tripping?

[HappyHippyDad]

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?

 

[static zap]

-No still confused- Sorry

 

[telectrix]

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]

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]

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

 

[Deleted account]

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]

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.

 

[westward10]

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.

 

[Deleted member 105166]

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

 

[Des 56]

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]

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]

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

 

[HappyHippyDad]

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

What's your thinking with this Tony? Just to see if greater than 30mA?

 

[davesparks]

.
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.

 

[Deleted member 105166]

What's your thinking with this Tony? Just to see if greater than 30mA?

I was trying to understand if an N-E fault, the actual imbalance between L&N and this may throw a pointer into the mix

 

[HappyHippyDad]

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.

I didn't disconnect the outgoing conductors. I've seen you have this discussion a few times with others Dave, although I can't remember the outcomes.
If for example I did disconnect the outgoing conductors and the RCD then tested ok would you be happy leaving it in place knowing that at any of the circuits it wil not trip when tested (i.e shows >40ms on x5, >300 on x 1 and >33ma on ramp test). Also, with the above in mind when an RCBO is added to one of the circuits it trips as expected when tested when the RCD in question does not (under exactly the same circumstances).

 

[davesparks]

I didn't disconnect the outgoing conductors. I've seen you have this discussion a few times with others Dave, although I can't remember the outcomes.
If for example I did disconnect the outgoing conductors and the RCD then tested ok would you be happy leaving it in place knowing that at any of the circuits it wil not trip when tested

Then how can you know that it is not an outgoing circuit which is causing the RCD to not trip rather than being a faulty RCD.
As the job is an EICR yes I would leave it as it is and code it accordingly, I expect either an FI or C2 code depending on the exact situation on site. The job is to report on the condition of the installation.
If I was testing for any other reason no I wouldn't just leave it as is but I would get to the bottom of the problem and repair or quote to repair as appropriate.

 

[Bobster]

Correct me if i'm wrong, but aren't there some models or RCBO that need testing at the output not at the screw? Something to do with where part of the sensing coil is located.

Seem to remember reading about it on here, but have not seen this first hand.

 

[westward10]

Memshield 2 but not the issue here.

 

[davesparks]

Correct me if i'm wrong, but aren't there some models or RCBO that need testing at the output not at the screw? Something to do with where part of the sensing coil is located.

Seem to remember reading about it on here, but have not seen this first hand.

Yes, it's the memshield 2 with the site fitted RCBO pod because the coil is in the pod so ends up after the screw terminal.