Hi Ze Reading

[sidekick]

2 bedroom annex at least 100m from main supply on 16mm 2 core swa with armour as earth and Ze of 2.42. Thinking easiest option to intall earth rod to improve.

 

[SJD]

Sounds a bit high a reading to be due to just the cable resistance? What is Ze at the supply origin? Perhaps poor or corroded connections of the armour?

 

[pc1966]

2C 16mm SWA should be 1.15 mOhm/m for R1 and 3.7mOhm/m for R2, so R1+R2 ought to be around 0.49 ohms. as @SJD says what you are seeing is a lot higher than realistically expected here so look for loose/corroded glands at the ends, or missing banjo+flying leads so it is depending on rusty/painted enclosures for the CPC.

If both ends are good and clean then it points to a more worrying aspect that it could be corroded armour underground somewhere, but that would usually take years to develop following puncture of the surrounding sheath. Also IR the cable on its own (both ends disconnected and wiped clean of any dust/damp), that might indicate water ingress somewhere if it looks low-ish.

More generally you won't get much of an improvement with an earth rod, often it is hard to get Ra below 50 ohm with most soils and that is not going to make a significant change to 2.4 ohms.

If after fixing it still is too high for the supply OCPD then fitting a delay RCD along with the MCB/fuse feeding it is another means of ensuring ADS times are mot and you still have a sporting chance of selectivity with end circuit RCD/RCBO at the annex CU.

You could use a rod and delay RCD if the armour is failing and cable otherwise OK, in effect making the annex a TT system with part of the sub-main on TN earthing (to the corroded segment) and the end of the system on TT using the rod at the annex. But you have to be able to meet ADS on the rod alone, hence the only realistic option is a 100mA or 300mA delay RCD as part of the supply protection.

 

[westward10]

Insulation test L to E long length of SWA

Insulation test live to earth = 0.7 ohms. This is a long length of buried swa cable (approx 100m) from mains in to submain. Test done with live and earth disconected at main and with switch off at sub main. Question - is this an accurate reading or am I misding something. Its a big job to...

www.electriciansforums.net

 

[DefyG]

Ze of 2.42.

Presume you mean Zs? so as per post#2 what is the Ze?

 

[sidekick]

Sounds a bit high a reading to be due to just the cable resistance? What is Ze at the supply origin? Perhaps poor or corroded connections of the armour?

Connections look all clean and tight with origin reading 0.16
Ins res L-E 3.37, N-E 4.27
and N-L 2.36 so not great

Sounds a bit high a reading to be due to just the cable resistance? What is Ze at the supply origin? Perhaps poor or corroded connections of the armour?

 

[DPG]

Connections look all clean and tight with origin reading 0.16
Ins res L-E 3.37, N-E 4.27
and N-L 2.36 so not great

Is this the same 100M cable as the one in the link in post #4?

 

[westward10]

So Ze is 0.16 and Zs is 2.42. Is this measured or calculated by continuity.

 

[sidekick]

Presume you mean Zs? so as per post#2 what is the Ze?

Thank you DefyG
Ze at origin 0.16, zs at annex csu 2.42, total zs on socket circuit 2.67

 

[sidekick]

Is this the same 100M cable as the one in the link in post #4?

yes

 

[DPG]

yes

Didn't you get an IR reading of 0.7M between two of the conductors?

 

[sidekick]

So Ze is 0.16 and Zs is 2.42. Is this measured or calculated by continuity.

measured

 

[DefyG]

So should be looking at a Zs reading of 0.16+0.49 = 0.65? (when energised, but presumably this is an old unused cable) so sticking with expected R1+R2 for now should be 0.49ohms?

I could understand high L-E and N-E measurement if corroded swa at gland or similar but that shouldn't affect L-N measurement which should be about 0.23ohms! unless something amiss with cable?

 

[sidekick]

Didn't you get an IR reading of 0.7M between two of the conductors?

I did. Have since gone back withnewly calibrated tester and done a new clean test. Its quite concerning that the reading is so different. The 63a mcb at origin has tripped out several times but not recently.

 

[sidekick]

So should be looking at a Zs reading of 0.16+0.49 = 0.65? (when energised, but presumably this is an old unused cable) so sticking with expected R1+R2 for now should be 0.49ohms?

I could understand high L-E and N-E measurement if corroded swa at gland or similar but that shouldn't affect L-N measurement which should be about 0.23ohms! unless something amiss with cable?

cable in ground approx 25 years

 

[westward10]

Do some continuity tests to establish where continuity is amiss.
R1+R2, R1+Rn, R2+Rn.

 

[DefyG]

Its quite concerning that the reading is so different.

What IR readings are you getting now?

 

[Julie.]

Ze o

Connections look all clean and tight with origin reading 0.16
Ins res L-E 3.37, N-E 4.27
and N-L 2.36 so not great

You don't appear to be clear with the problems here.

You talk about insulation resistance of 2.36 to 4.27 - is this Mohm?

This is irrelevant to the Zs

You don't appear to know the differences between insulation resistance, Ze and Zs.

If you do have Ze of 0.16 ohm at the feed end of the cable and ~2.5 ohm at the load end, there is absolutely something wrong with the cable or terminations.

I would expect a r1 + r2 of 0.5 ohms or thereabouts.

You need to find this fault, not cover it up by adding rods or anything else.

You need to measure the r1 + r2, the r1 + rn etc. This will show if you have a cable fault. And will identify which conductor is the problem, if they all read 0.5 ohms (less for r1 + rn) then the fault is between the cable feed end and the header. - most likely a bad connection in the earth path.

 

[westward10]

Thank you DefyG
Ze at origin 0.16, zs at annex csu 2.42, total zs on socket circuit 2.67

The Zs for the socket circuit is not relevant your problem is the submain.

 

[sidekick]

What IR readings are you getting now?

Ins res L-E 3.37, N-E 4.27
and N-L 2.36