Insulation resistance confusion

[Calebp43]

Been to a fault finding job and lights where tripping the rcbo. After further investigation we found that it was a blue strapper cable (old wiring) from one switch to another using 3 core. Let's call this cable 1.
After hacking some of the cable out the wall we found that the other flat t and e had a partially broken live conductor. Let's call this cable 2.
So we fixed that but for some reason after we fixed cable 2 we then did an IR on cable 1 on all the 3 cores individually to the CPC in cable 1 and was getting 0.01, 0.02 and 0.03 ohms?!
it's worth noting that we disconnected cable 1 from the other switch and separated all the cores, so we where just testing that cable.
we put everything back together to see if it worked and it did. The rcbo didn't trip even with the loads.
the wall was like a 15 foot high wall that the cables where chased into and the customer was watching the whole time and didn't was us to keep chasing out if it wasn't necessary as it was working.
I left that job very unsatisfied and can't seem to grasp how the cables with such a low IR reading to earth isn't tripping the rcbo. Any thoughts? Thanks.

 

[westward10]

Ohm or Meg ohm.

 

[DPG]

Been to a fault finding job and lights where tripping the rcbo. After further investigation we found that it was a blue strapper cable (old wiring) from one switch to another using 3 core. Let's call this cable 1.
After hacking some of the cable out the wall we found that the other flat t and e had a partially broken live conductor. Let's call this cable 2.
So we fixed that but for some reason after we fixed cable 2 we then did an IR on cable 1 on all the 3 cores individually to the CPC in cable 1 and was getting 0.01, 0.02 and 0.03 ohms?!
it's worth noting that we disconnected cable 1 from the other switch and separated all the cores, so we where just testing that cable.
we put everything back together to see if it worked and it did. The rcbo didn't trip even with the loads.
the wall was like a 15 foot high wall that the cables where chased into and the customer was watching the whole time and didn't was us to keep chasing out if it wasn't necessary as it was working.
I left that job very unsatisfied and can't seem to grasp how the cables with such a low IR reading to earth isn't tripping the rcbo. Any thoughts? Thanks.

Are those figures definitely Ohms, or are they Megohms?

 

[DPG]

Ohm or Meg ohm.

Ooh snap!

Assuming, since you are doing an IR test, that the figures are in Megohms, then 0.02M would give 12mA and so would not trip a 30mA RCD.

 

[Calebp43]

Sorry guys it's mega ohms.

 

[Calebp43]

Ooh snap!

Assuming, since you are doing an IR test, that the figures are in Megohms, then 0.02M would give 12mA and so would not trip a 30mA RCD.

How did you get to 12mA?

 

[Intoelectrics]

How did you get to 12mA?

ohms law.

 

[Calebp43]

ohms law.

Thought so but it came to 0.115 but yeah when you round it up makes sense.

What's confusing though is I've been to callouts before for a fault and the fault trips the RCD at 0.10Mohms

 

[DPG]

Thought so but it came to 0.115 but yeah when you round it up makes sense.

What's confusing though is I've been to callouts before for a fault and the fault trips the RCD at 0.10Mohms

No, it's 0.0115 - you are an order of magnitude out!

0.115 would be 115mA and would definitely trip an an RCD.

It's vital you get your units and values correct. The two errors you have made mean your results are wildly out.

 

[Intoelectrics]

Thought so but it came to 0.115 but yeah when you round it up makes sense.

What's confusing though is I've been to callouts before for a fault and the fault trips the RCD at 0.10Mohms

The leakage from that value alone may not trip the RCD, but if there is leakage from things connected on the circuit then there may well be enough combined leakage to trip at the time of testing.

 

[Intoelectrics]

I left that job very unsatisfied and can't seem to grasp how the cables with such a low IR reading to earth isn't tripping the rcbo. Any thoughts? Thanks

So have you not done any further investigation, or confirmed your results? Even if the RCBO is not tripping low IR readings like these should be investigated further (assuming the results are as you stated).

 

[Lucien Nunes]

Those IR readings are in the naughty zone where the low resistance is actually a hazard in itself, not just a tell-tale that alerts the tester of the need for further investigation.

Readings of a few megohms are useful indicators of possible cable damage or damp but the lowered resistance itself isn't normally hazardous. Even hundreds of kilohms, although obviously indicative of a fault, might persist for a long time without giving rise to danger. But at 20kΩ, the power dissipation at the insulation breach is a few watts and if it's localised could cause overheating, progressive charring to insulation etc. RCBO protection or not, I would not energise at that low IR.

It's actually quite rare to be so low unless the cable is fire-damaged, chewed up and surrounded by wet mud or similar. With all cores leaking to CPC I could imagine a completely water-filled JB, but not a T+E in the wall that has had a swipe with a spread's trowel. You would be hard pressed to recreate a 10kΩ fault in T+E indoors, just by beating it up or putting tools and fixings through it. A dead short, or a >1MΩ fault, is much more likely than 10kΩ.

It makes me wonder whether the IR readings are correct or whether there is some other explanation for them?

 

[Calebp43]

So have you not done any further investigation, or confirmed your results? Even if the RCBO is not tripping low IR readings like these should be investigated further (assuming the results are as you stated).

That's a great point about other things being connected and adding up to it tripping the breaker.

What you said about investigating - we know the fault is in the wall. We narrowed it down to that. It's a massive wall like 15-20 foot high but the customer didn't want us chasing out the wall if the circuit is working.

We did explain to the customer that we wasn't too happy but we understand not everyone is gonna want work like that doing. He understands that the fault might get worse and trip it so if it comes to that we will most likely run a new cable in trunking for him.

Also I'm an apprentice I don't call the shots either way. Maybe I should've mentioned that so my bad. (was working with another sparky but he very recently qualified and was also a bit confused by this)

 

[Calebp43]

Those IR readings are in the naughty zone where the low resistance is actually a hazard in itself, not just a tell-tale that alerts the tester of the need for further investigation.

Readings of a few megohms are useful indicators of possible cable damage or damp but the lowered resistance itself isn't normally hazardous. Even hundreds of kilohms, although obviously indicative of a fault, might persist for a long time without giving rise to danger. But at 20kΩ, the power dissipation at the insulation breach is a few watts and if it's localised could cause overheating, progressive charring to insulation etc. RCBO protection or not, I would not energise at that low IR.

It's actually quite rare to be so low unless the cable is fire-damaged, chewed up and surrounded by wet mud or similar. With all cores leaking to CPC I could imagine a completely water-filled JB, but not a T+E in the wall that has had a swipe with a spread's trowel. You would be hard pressed to recreate a 10kΩ fault in T+E indoors, just by beating it up or putting tools and fixings through it. A dead short, or a >1MΩ fault, is much more likely than 10kΩ.

It makes me wonder whether the IR readings are correct or whether there is some other explanation for them?

They are 100% correct. Do you think that this is dangerous?

This was my fear and when we spoke to our boss (who has been a spark for 30 years) he didn't say too much. He never really does and for all I know he's gone back but that's why I come to these forums because I always get very useful info on things when I'm confused about something. Don't like asking the boss because I struggle understanding and learn much better when things are written out and I can go over it as many times as I want.

 

[Intoelectrics]

That's a great point about other things being connected and adding up to it tripping the breaker.

What you said about investigating - we know the fault is in the wall. We narrowed it down to that. It's a massive wall like 15-20 foot high but the customer didn't want us chasing out the wall if the circuit is working.

We did explain to the customer that we wasn't too happy but we understand not everyone is gonna want work like that doing. He understands that the fault might get worse and trip it so if it comes to that we will most likely run a new cable in trunking for him.

Also I'm an apprentice I don't call the shots either way. Maybe I should've mentioned that so my bad. (was working with another sparky but he very recently qualified and was also a bit confused by this)

Ok, well its good that you are coming to these forums to learn. Sometimes faults like these can be difficult to pin point.
As an apprentice I would suggest that its not your fault. But as Lucien described in his post faults like this need to be rectified, not just ignored just because the RCBO doesn't trip.

It's the responsibility of an electrician to ensure that an installation is left in a safe order. This may mean rectification of a fault or isolation of a circuit until the fault is rectified.

If the fault is there and the results are confirmed as per your readings, then I maybe would have isolated the circuit and explained to the customer the situation and the potential danger. If the customer then decides to ignore your advice/recommendation and then switches the circuit back on, then well, that's their call.

The bottom line is, based on your current position, the fact that you are an apprentice, then I would suggest that its the call of the senior electrician to make or if not then the main person responsible (your boss).