Zs high at db lower in field.

[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??????????

 

[Mr Mark Sparks]

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

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

 

[Spudnik]

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!

 

[Mr Mark Sparks]

where are you lenny

 

[KNIPEX]

no bonds go back to same db, this is going to be intresting one i think, quite a few of us looking for answers on this

 

[lewlec]

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?

 

[KNIPEX]

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

 

[lewlec]

ah, i see... not sure what to suggest then

 

[Guest123]

Hey.

What is it all wired in?? T+E, galv conduit, swa etc??

 

[KNIPEX]

the submain is in swa with supp earth.

the install is in t+e plastic conduit.

 

[Guest123]

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.

 

[KNIPEX]

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??

 

[Spudnik]

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.

 

[Guest123]

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

 

[KNIPEX]

it would be zs as it miles away from incomer thanks for the replies guys, an intresting one to say the least.