Strange Zs readings!!!!

[apprentice87]

Hi all,

Here is a good one...

I have been installing [over some time] a second DB [fed from a first one, NOT direct from the origin] in a large steel framed shed.

Now, this takes some explaining...

DB one is bolted to the steel work. DB two is on a brick wall [no parallel paths]

Zdb at the first board is .23 ohms

R1 + R2 for the cable BOTH from tables [and also from measurement] is about .266 ohms.

So, you would be looking at a Zs at board two of near enough .5 ohms.

Sure enough, this is what i measured.

SINCE THEN, I HAVE BONDED THE SECOND BOARD TO THE STEELWORK.

A stanchion was 6 inches away, so, i thought i might as well supplementary bond the thing, as if you were leaning on them both and there was an earth fault and the "proper" earth had a high resistance, you would serve quite well as an earth conductor between board and shed.

Anyway, today though, whilst doing something else, i re-measured R1 + R2 This time is it a lot lower, at only 0.9 ohms. Now you would expect that, as now the shed frame forms a "parallel path"

Problem is though, the Zs, at the second board, remains about the same as it was before, WITHOUT the parallel path.

How could this be???

Well and trully confused!!!!!

john....

 

[steptoe]

poor connections on your leads.

---------- Post Auto-Merged at 23:13 ---------- Previous post was made at 23:12 ----------

BTW, R1 R2 doesnt include parallel paths! so you are measuring it wrong!

 

[apprentice87]

Hi Steps,

I do not understand...... If say two items are linked together with SWA and they are also both bolted to a metal frame, then, if you attempt to measure R1 + R2, you will get a funny reading as the metal frame is in parallel with your cable armour used as your cpc, unless you want to disconnect the gland at one end that is.

More confused now, as i know you know what you are talking about!!!!

john...

 

[Technician]

John; In your original post should the result of the test

be 0.09 ohms?

You said it was a lot lower and 0.9 is greater than 0.5 Ohm.

and 0.23Ohm.

 

[apprentice87]

Hi Technician!!!!!

Well spotted!!!! Teach me to write things when i am knackered!!!!!

john..

Found out some more VERY interesting stuff today though!!! I will add to my post later, when i get back, [Got to go and finish off!!!]]

john

 

[kelvfoozinda]

Is the steel framed shed an external building? If so you shouldn't be exporting the earth via swa anyway.

 

[park1]

Hi all,Here is a good one...

I have been installing [over some time] a second DB [fed from a first one, NOT direct from the origin] in a large steel framed shed.

Now, this takes some explaining...

DB one is bolted to the steel work. DB two is on a brick wall [no parallel paths]

Zdb at the first board is .23 ohms

R1 + R2 for the cable BOTH from tables [and also from measurement] is about .266 ohms.

So, you would be looking at a Zs at board two of near enough .5 ohms.

Sure enough, this is what i measured.

SINCE THEN, I HAVE BONDED THE SECOND BOARD TO THE STEELWORK.

A stanchion was 6 inches away, so, i thought i might as well supplementary bond the thing, as if you were leaning on them both and there was an earth fault and the "proper" earth had a high resistance, you would serve quite well as an earth conductor between board and shed.

Anyway, today though, whilst doing something else, i re-measured R1 + R2 This time is it a lot lower, at only 0.9 ohms. Now you would expect that, as now the shed frame forms a "parallel path"

Problem is though, the Zs, at the second board, remains about the same as it was before, WITHOUT the parallel path.

How could this be???

Well and trully confused!!!!!

john....

Probably down to the test currents used the differing effects with SWA and the reliability of any fortuitous paths.

 

[Technician]

 

[steptoe]

Hi Steps,I do not understand...... If say two items are linked together with SWA and they are also both bolted to a metal frame, then, if you attempt to measure R1 + R2, you will get a funny reading as the metal frame is in parallel with your cable armour used as your cpc, unless you want to disconnect the gland at one end that is.

More confused now, as i know you know what you are talking about!!!!

john...

in theory you should disconnect one end of the cable, just the same as if you were taking the measurement on a T&E radial,

leaving it connected, is, as you say, using the steelwork as a path, you have not actually proved the continuity of the armour at all.

 

[apprentice87]

Hi Steps,

I had another go today, only this time i disconnected the "far" end entirely. Discovered a lot of other interesting things today too!! I learnt a lot!!! I will write a bit on this later as i have just this second come in. I am knackered, was REALLY hot here today!!

john..