Discuss Zs high at db lower in field. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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K

KNIPEX

hi guys, this is a good one, a db which is quite away from main incommer so off busbar to mccb etc.

anyway the zs at the db is quite high say 0.59

then out in the field the sockets and lighting circuits and ringmains are a much lower zs, for example 0.35 on lighting and other various sockets.

this sounds very odd to me and not right anyone else come across this or got any advice??????????
 
at first wondered if the outlets were getting back through a paralel path sup bonds metal work etc but not sure why this wouldnt bring the zs at the cu down to.
unless the bonds go back to different cu.
now im just rambling:confused:

i installed some site temps a while ago and got 114ohms direct to stake at main cu then fed toilet block with 3core single phase supply this had zs of40ohms(through cable cpc), this made sense untill i redone zs at 1st cu and it was still 114 ohms maybe we can both get an answer
 
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if i can chip in also, i did a PIR on a boarding kennels and the voltage reading was getting higher, the further i was from the incoming!

I would also appreciate any input too!
 
HI EVERY ONE, FIRST POST SO TAKE IT EASY!
Just a thought, could it be that there is a neutral to earth fault near one of the final circuits, causing some sort of pme system???? therefore getting lower readings? what r u thoughts?
 
HI EVERY ONE, FIRST POST SO TAKE IT EASY!
Just a thought, could it be that there is a neutral to earth fault near one of the final circuits, causing some sort of pme system???? therefore getting lower readings? what r u thoughts?

no mate as this is new installation and all circuits have been fully tested :)
 
Ok heres my theory.

When your measuring Zs at the dis board the main switch is open and the seperately installed CPC with the sub-main is disconnected yes?? so the only earth path you are measuring is via the seperately installed CPC.

Where as when you are measuring Zs at socket outlets and lighting points, all earths are connected i.e CPC, armourings, water, gas and structural steelwork bonds therefore parallell paths exist and the resistance is greatly decreased giving you the lower readings on the final circuits.

IMO this is the only thing it can be.

Hope this helps.
 
Ok heres my theory.

When your measuring Zs at the dis board the main switch is open and the seperately installed CPC with the sub-main is disconnected yes?? so the only earth path you are measuring is via the seperately installed CPC.

Where as when you are measuring Zs at socket outlets and lighting points, all earths are connected i.e CPC, armourings, water, gas and structural steelwork bonds therefore parallell paths exist and the resistance is greatly decreased giving you the lower readings on the final circuits.

IMO this is the only thing it can be.

Hope this helps.


yeah seems that way mate, still complies though doesnt it, can see anything wrong with it myself, can you??
 
As long as the results are within the limits.

From what Lenny is saying, ie parallel paths, do a ZE with bonds connected and see what reading you get.

That would then confirm what the man said. Or not.:D
 
No.

From the values you've described I'm pretty sure they all fall under max permissable values.

As long as you've checked your measured values against the tabulated values and they comply then it's fine.

Cheers
 
it would be zs as it miles away from incomer thanks for the replies guys, an intresting one to say the least.
 
How much higher is it getting then??

What equipment have you got in the circuits then mate?? lots of electronic equipment could do it

It started at around 235, varied in between and then ended up nearer 245 roughly at the furthest point.

There was very little load on the system during the day.

Even when it is loaded, its only lighting and heating.

Done many a PIR, although not like this, and never had this prob before.
 
Could be a capacitor in a light fitting breaking down and shunting more voltage into the system than it should.

I got brain fog now:(
 

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