EICR - high readings

[dan7202]

I am part way through a condition report for a customer.
The house is generally in good condition but the electrical installation is quite old.
Although the rings are properly installed as ring final circuits, the 2.5 mm t and e cable is stranded and the Cpc appears to be 1 mm rather than 1.5 mm.
I expect the r2 readings to be proportionately higher because of this but they are between 10% and 40% higher than I would have expected.
When checking behind sockets all the connections seem solid but the cables do look a bit worn (sheathing looks tired )and have possibly been subject to damp.
The r1/r2 crossover readings are also higher than expected. And I have based the expected readings for these on the measured R1 and R2 readings.
The installation seems all over the place and generally tired, there are other issues such as broken neutral and faulty shower connections which I have found. Most of the circuits have low IR readings as well which suggest to me that there is a general degradation of the cables.
Am I missing anything in terms of other possible causes.?

Any suggestions welcome.

 

[Deleted member 72666]

What values are you getting for IR?

 

[telectrix]

your r2 readings should be 2.5 x r1 (and rN) for a 2.5/1.0mm circuit.

 

[Guitarist]

Definitely crossing L and CPC correctly?

 

[GLENNSPARK]

Definitely crossing L and CPC correctly?

yes but if he has deemed himself competant to carry out ECRs...then he should know what to look for when going round the points on incorrect cross connecting of ring...

 

[GLENNSPARK]

O/P needs to check for spurs off spurs....and rings within a ring...

both will give different readings to what you would generally expect....

 

[Lucien Nunes]

How many strands are in the live conductors and the CPC? Are the strands of the CPC the same gauge as the lives? Are they tinned?

Stranded T+E (less than 4mm²) used for ring mains is usually imperial 7/.029, not 2.5mm². The lives would be 2.93mm² and if the CPC is 3/.036 as prescribed by the 14th ed this would be 1.93mm², the ratio being 1.5:1.

However there were other sizes of CPC used in 7/.029 and it could be 3/.029 or even 1/.044, which at 0.98mm² makes the ratio nearly 3:1. If the loop impedance and adiabatics are OK then the (relatively) high reading might be acceptable.

If you are not familiar with distinguishing between imperial and metric sizes, the best way to be certain of the CSA is to measure a strand with a micrometer. As far as degradation goes, low IR is rarely the result of undamaged cables simply aging, but do look out for green goo that would indicate breakdown.

 

[GLENNSPARK]

How many strands are in the live conductors and the CPC? Are the strands of the CPC the same gauge as the lives? Are they tinned?

Stranded T+E (less than 4mm²) used for ring mains is usually imperial 7/.029, not 2.5mm². The lives would be 2.93mm² and if the CPC is 3/.036 as prescribed by the 14th ed this would be 1.93mm², the ratio being 1.5:1.

However there were other sizes of CPC used in 7/.029 and it could be 3/.029 or even 1/.044, which at 0.98mm² makes the ratio nearly 3:1. If the loop impedance and adiabatics are OK then the (relatively) high reading might be acceptable.

If you are not familiar with distinguishing between imperial and metric sizes, the best way to be certain of the CSA is to measure a strand with a micrometer. As far as degradation goes, low IR is rarely the result of undamaged cables simply aging, but do look out for green goo that would indicate breakdown.

quite right lucien...

depends on how much fault it has been subjected to....as well as the usual (temprature, installation method, etc)..

when replacing old boards its worth while looking at how much splash is in the fuse carrier.......this can give an indication of whether the install has had a hard life...

 

[dan7202]

Thanks for the replies
Yes the crossovers were correct and I do know how to recognise if they aren't.
r2 values are generally double the r1 values with the exception of one circuit where it was 2.5 times the r1 value.

IR readings are all around the 500 Mohm value.

 

[dan7202]

And, there are no signs at the sockets of spurs or rings within rings but there are limitations with the inspection regarding the building fabric and floorboards as the person commissioning the inspection is a prospective buyer and not the owner.

 

[Guitarist]

How many strands are in the live conductors and the CPC? Are the strands of the CPC the same gauge as the lives? Are they tinned?

Stranded T+E (less than 4mm²) used for ring mains is usually imperial 7/.029, not 2.5mm². The lives would be 2.93mm² and if the CPC is 3/.036 as prescribed by the 14th ed this would be 1.93mm², the ratio being 1.5:1.

However there were other sizes of CPC used in 7/.029 and it could be 3/.029 or even 1/.044, which at 0.98mm² makes the ratio nearly 3:1. If the loop impedance and adiabatics are OK then the (relatively) high reading might be acceptable.

If you are not familiar with distinguishing between imperial and metric sizes, the best way to be certain of the CSA is to measure a strand with a micrometer. As far as degradation goes, low IR is rarely the result of undamaged cables simply aging, but do look out for green goo that would indicate breakdown.

I always find it amusing that you can show a customer very low IR readings and yet they won't believe that there's anything wrong, yet show them the green goo (even if figures are good) and they want a rewire there and then!

 

[dan7202]

Thanks for the replies
The ir values are all around the 500 mark
Resistance for the r2 are about double the r1
I have definitely done the crossovers correctly and know the signs if I haven't .
There are no signs of spurs off spurs at the sockets but I am limited with the building fabric and floorboards as the inspection is for a potential buyer and not the current owner

 

[Lucien Nunes]

IR readings are all around the 500 Mohm value

Are these the ones you're describing as low?

10 yards of new cable terminated in a new fitting on a dry day should top that easily, but if the weather round your way is anything like it is here, 500MΩ is pretty good for an old circuit that will have a fine film of dust and dirt over all the insulation in the accessories, holding the moisture and encouraging leakage. I'd be happy with that on a new install.

With reference to the resistance of the CPC vs the lives, you'll have to get your mike out and measure. You didn't answer my question about the number of strands but it sounds like you might have a mix of cables, perhaps mostly 7/.029 with a bit of 2.5 thrown in.

FWIW I do like 7/.029 rings, it has that extra bit of capacity and yet it's more flexible than 2.5. Would never have looked right in blue and brown though.

 

[Pete999]

Don't you mean R1 and R2 as opposed to r1 and r2 they are different you know

- - - Updated - - -

Don't you mean R1 and R2 as opposed to r1 and r2 they are different you know

 

[Pete999]

Sorry about that pressed the send button twice, nugget

 

[Lucien Nunes]

And I'm not helping with my R1 reference either, I'll change that to avoid confusion