High Zs reading

[Jacko]

Hello,

looking for some advice on 2 things please.

Ive tested a ring main in a house with RCD protection, the results ive had are

L-L .43
N-N . 41
E-E 1.17

I had a look for a loose connection but can’t find anything, how high does the CPC reading need to be before you would consider it ‘to high’?

the second question is regarding the same circuit, they have a fuse spur fed from one of the sockets and then have 6 x double sockets after the fuse spur. The last 3 are out door and the last socket on the ‘radial’ gives a Zs reading of 1.64.

Am I right in saying max Zs should be 1.08? Is the high reading due to the distance on the run after the fuse spur?

Is the reading acceptable?

the reading on all sockets were lower than .71 until the last 3 outside which are run using SWA cable.

thanks

 

[stevethesparks]

I think the cpc in a 2.5 T&E was uprated to 1,5 in the early 80's but I'm sure others with better knowledge will confirm

 

[Julie.]

It says on the inside of my EICR certificate book that 1.08 is the max Zs value allowed on a type B 32 amp MCB. Is that correct?

if the radial off the ring via the spur is higher than 1.08 but less than the allowed value after a 13amp fuse, which value would you write on the test form?
Thanks

Yes, that would be the correct limit for the ring and the spur up to the fuse.

From the fuse onwards, it isn't protected by the mcb, but by the fuse, so you need to use the appropriate value for that, think it's about 80% of 2.3 ohm, I haven't got my book around.

As for the eic/eicr depends on your software, but I would add an additional line below the rfc entry (no circuit in the cu) detailing the radial off the rfc as a seperate entity but not from the cu mcb

The use of 2.5/1.0 was around the metric versions of the 14th I think, way more than 15 years ago unless there was another phase of use

 

[Jacko]

Hi - the max loop impedance for BS1362 fuses can be found in BBB Table 41.2 (d) . This gives 2.3 Ohms for 13A, so 80% is 1.84 Ohms. So looks ok .

Excellent. Would you write that result on the test form and mention the fuse spur somewhere? Thank you

 

[Jacko]

Yes, that would be the correct limit for the ring and the spur up to the fuse.

From the fuse onwards, it isn't protected by the mcb, but by the fuse, so you need to use the appropriate value for that, think it's about 80% of 2.3 ohm, I haven't got my book around.

As for the eic/eicr depends on your software, but I would add an additional line below the rfc entry (no circuit in the cu) detailing the radial off the rfc as a seperate entity but not from the cu mcb

The use of 2.5/1.0 was around the 14th I think, way more than 15 years ago unless there was another phase of use

Thank you for your help.

 

[Jacko]

I think I should look again for the loose connection on the CPC, I’m pretty sure the cable is 2.5-1.5.

does the result have to be within 1.67 or is it allowed to be slightly over? I was told 1.67 is what we would expect to see so when would you start to look into it?

 

[westward10]

Yes you can give some leeway but if your cpcs are 1.5/1.5 then your reading is way above what it should be.

 

[Jacko]

Yes you can give some leeway but if your cpcs are 1.5/1.5 then your reading is way above what it should be.

Should I be looking for Anything other than a loose connection ?

 

[Julie.]

I think I should look again for the loose connection on the CPC, I’m pretty sure the cable is 2.5-1.5.

does the result have to be within 1.67 or is it allowed to be slightly over? I was told 1.67 is what we would expect to see so when would you start to look into it?

Typically MFTs are +/- 0.05 ohm.

So if the calculated value was say 0.41 x 1.667 = 0.683 ohm. The it could be exactly this and measure anything between 0.633 and 0.733

In reality the values could be a bit wider to account for bad contacts etc.

Switching the socket on/off a few times, and plugging/unplugging can bring it down

So it's a matter of what you think is reasonable

 

[westward10]

Should I be looking for Anything other than a loose connection ?

Had two circuits like this couple of weeks ago and both were terminal screws on the green/yellow sleeving and not the conductor.

 

[Wilko]

Excellent. Would you write that result on the test form and mention the fuse spur somewhere? Thank you

If the fused spur was on Cct 4 say, then I put it under that as Cct 4A and put a short explanation in the note section.