ZS

[DRC]

Hi all

wondering if you can help me when taking a reading for zs does the reading have to be the the same as ZE+(R1 + R2)?

The reason I am asking is my boss insists it should be the same, but i think that as long as the reading is under 80% value then all is ok. Just wondering who is right and who is wrong.

 

[alpha spark]

In theory yes but practically not I had a argument with a previous employer I run a 6mm swa to a overdoor heater in a shop over a length of 50 + meters when it came to testing the zs was really low he pulled me and said I was making up the readings in which I replied the apprentice did the full set of tests in front of me and if I was making it up do u honestly think I would put a reading so low !

 

[DRC]

So in theory they should be about the same but wouldn't parallel paths affect the readings?

 

[apprentice87]

Hi there,

You are......

Firstly, you are EXACTLY right when you say that the MEASURED reading should not be more than 80% of the CALCULATED one. This as you will be well aware, is to compensate for the fact that in use, especially under fault conditions, the cables will get hotter maybe 70 or 90c or more [the R1 + R2 given in tables is given at 20C] and therefore the resistance will increase.

Be aware though, that you might well measure Ze, add your calculated R1 + R2, work out what 80% of this is, and go to check that all is well with your meter, and find a LOWER figure than you were expecting, due to the existance of parallel earth paths that will reduce R2.

I always try to calculate what Zs will be if I can, and NOT rely on the existance of parallel paths at all, as, after all, one day these "accidental" earth return paths might be removed. For example, someone could demolish or remove parts of a steel framed shed that was acting as a mega CPC for half the installation, leaving behind just the cable armour to act as CPC for distribution circuits for example, instead of the big thick steel beams that the shed was composed of, and then the Zs of what is left could well increase to unacceptable levels....

john...

 

[steptoe]

measured Zs is almost always lower IME, remember though, you always use the highest reading.

 

[apprentice87]

Steps will be along in a minute just to give his definitive advice.

Believe me, you will be able to rely on what Steps says..... [Cos he KNOWS his stuff!!!!]

john..

---------- Post Auto-Merged at 11:24 ---------- Previous post was made at 11:22 ----------

Ha ha, I did tell you!!!!

Steps is ALWAYS right..... He knows his stuff big style!!!!!

john....

 

[steptoe]

Have I missed something John?

BTW, small point, the 80% relates to permitted values, nothing to do with calculated, your call'd or measured readings should be <80% to allow for increase in temp during fault conditions.

 

[apprentice87]

Hi Steps,

No i do not think you missed anything, all sounded good to me!!...

When you said "BTW, small point, the 80% relates to permitted values, nothing to do with calculated, your call'd or measured readings should be <80% to allow for increase in temp during fault conditions"

It was just a simpler [and better i think] way of explaining the bit I said about "you are EXACTLY right when you say that the MEASURED reading should not be more than 80% of the CALCULATED one. This as you will be well aware, is to compensate for the fact that in use, especially under fault conditions, the cables will get hotter maybe 70 or 90c or more [the R1 + R2 given in tables is given at 20C] and therefore the resistance will increase"

Hope all is well up where you are!!!

john...

 

[Doc Hudson]

Hi all wondering if you can help me when taking a reading for zs does the reading have to be the the same as ZE+(R1 + R2)?

The reason I am asking is my boss insists it should be the same, but i think that as long as the reading is under 80% value then all is ok. Just wondering who is right and who is wrong.

You can tell your boss he does not know what he is talking about. As has been pointed out parallel paths play a significant part. Remember you take Ze with all bonding conductors disconnected, so you are only reading the external earth path. R1+R2 is a dead 'resistance' reading, using a DC test voltage, down a pair of conductors that are isolated from all supplementary and equipotential bonding conductors. Zs is a live 'impedance' measurement using 230v AC along a line conductor and CPC and with all bonding in place. Zs is a 'cold' test when the cables are not at their normal operating loading temperature. This is why the tabulated and measures readings use the 80% rule of thumb. Zs is often less than Ze + (R1+R2), Occasionally it can be almost equal to Ze + (R1+R2), But it should never be greater than Ze + (R1+R2)

Doc H.

 

[DRC]

Thanks a lot guys

Good to no i am on the right track

 

[Doc Hudson]

Thanks a lot guys Good to no i am on the right track

Glad the forum has been of some help, you can use the 'like' feature, bottom right of each post, or the reputation feature 'Star' at the bottom left of each post to thank any particular members who have helped you. Remember all of our members give up their time and share their knowledge on a voluntary basis.

Doc H.

 

[m4tty]

Glad the forum has been of some help, you can use the 'like' feature, bottom right of each post, or the reputation feature 'Star' at the bottom left of each post to thank any particular members who have helped you. Remember all of our members give up their time and share their knowledge on a voluntary basis.Doc H.

Reputation star .... Are you in canoes competition too Lol ?

 

[DRC]

:innocent