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]

Is this an old install with 1.0mm cpc?
Roughly for the figures given for the L&N, cable length would calculate to approx 62 metres
the cpc should calculate to approx 0.72 for a 1.5mm and 1.08 for a 1.0mm
you may even get a lower reading on the cpc if there are any parallel earth paths.

 

[westward10]

Agree with above those readings indicate 1.0/1.0 cpcs if they are not then the r2 reading is way too high.

 

[westward10]

Quick rule of thumb to see if the r2 is approximately correct is to times the r1 by 1.67 for 2.5/1.5 cable, 2.5 for 2.5/1.0 cable and 2.67 for 4.0/1.5 cable.

 

[Julie.]

The generic values for the cpc compared to the live conductors is 'live value' x ( live x-section/cpc x-section)

So
2.5/1.5 = 1.667 as above
2.5/1.0 = 2.5 ( with 0.41 gives 1.025)

How are you calculating the Zs as 1.08?

In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the r1 + r2 for all sockets on the ring, but the radial off the ring would not be this value.
It would be this value plus the measured r1 + r2 of the radial portion.
Whatever value you get for the zs on the radial may be above the allowed value for the OCPD feeding the ring, but below the allowed value for the 13A fuse

 

[Julie.]

The generic values for the cpc compared to the live conductors is 'live value' x ( live x-section/cpc x-section)

So
2.5/1.5 = 1.667 as above
2.5/1.0 = 2.5 ( with 0.41 gives 1.025)

How are you calculating the Zs as 1.08?

In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the r1 + r2 for all sockets on the ring, but the radial off the ring would not be this value.
It would be this value plus the measured r1 + r2 of the radial portion.
Whatever value you get for the zs on the radial may be above the allowed value for the OCPD feeding the ring, but below the allowed value for the 13A fuse

Sorry, just realised I got that wrong:

"In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the r1 + r2 for all sockets on the ring"

Should be
In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the Zs for all sockets on the ring.

It would be this value plus the measured r1 + r2 of the radial portion for the Zs at the end of the radial.

Typo unfortunately, I knew what I meant to say... oops

That and I am an idiot

 

[loz2754]

Sorry, just realised I got that wrong:

"In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the r1 + r2 for all sockets on the ring"

Should be
In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the Zs for all sockets on the ring.

It would be this value plus the measured r1 + r2 of the radial portion for the Zs at the end of the radial.

Typo unfortunately, I knew what I meant to say... oops

That and I am an idiot

Hi Julie. I think you may also have got your lower and upper cases mixed up...
Sorry to be such a pedant.

 

[Julie.]

Hi Julie. I think you may also have got your lower and upper cases mixed up...
Sorry to be such a pedant.

Yeah, I started to follow the terminology used in the original post, then realised I shouldn't have.

Bad day today

Walking boots leaked, then slipped on the ice and snow and slid down the hill like a clown, much to my husband's amusement!

Hasn't got any better since then!!!

 

[loz2754]

Yeah, I started to follow the terminology used in the original post, then realised I shouldn't have.

Bad day today

Walking boots leaked, then slipped on the ice and snow and slid down the hill like a clown, much to my husband's amusement!

Hasn't got any better since then!!!

Wish we had some snow.

 

[Julie.]

Wish we had some snow.

So did I until I stepped onto ice completely covered in snow!

 

[loz2754]

So did I until I stepped onto ice completely covered in snow!

Ouch! Hope you didn't get hurt too badly.

 

[Wilko]

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.

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 .

 

[Julie.]

Ouch! Hope you didn't get hurt too badly.

Pride only

 

[Jacko]

The generic values for the cpc compared to the live conductors is 'live value' x ( live x-section/cpc x-section)

So
2.5/1.5 = 1.667 as above
2.5/1.0 = 2.5 ( with 0.41 gives 1.025)

How are you calculating the Zs as 1.08?

In the ring r1 + r2 should be (R1+R2)/4
This should be added to the Ze value to give the r1 + r2 for all sockets on the ring, but the radial off the ring would not be this value.
It would be this value plus the measured r1 + r2 of the radial portion.
Whatever value you get for the zs on the radial may be above the allowed value for the OCPD feeding the ring, but below the allowed value for the 13A fuse

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

 

[Jacko]

Is this an old install with 1.0mm cpc?
Roughly for the figures given for the L&N, cable length would calculate to approx 62 metres
the cpc should calculate to approx 0.72 for a 1.5mm and 1.08 for a 1.0mm
you may even get a lower reading on the cpc if there are any parallel earth paths.

I’m not there now but I think it was 2.5/1.5 cable but you have got me thinking now.

there was 2 rings in the house, the other readings were
L .54
N.53
E.91

that looks like what I would expect for 2.5/1.5 cable. The house was built 14/15 years ago

 

[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.