What would you do??

[mark80]

Hi,
Just a quick question i was thinking about today. Say if your measured ze was over the maximum value. Say for arguments sake a tncs was 0.5ohms.
Would

1) It be fine leaving it as this value aslong as all circuits fall within maximum Zs
or
2) Install an earth rod and make the installation TT, as with the ze being over the maximum value it can not be classed as a stable earth???

Aint got the regs to hand to check if theres anything in on this, and just wondered what peoples view on this would be

 

[sparks1234]

TT is an option, maybe worth calling the supplier though and see if there is a reason, would be ok on a TN-s system, max 0.8ohm

 

[i=p/u]

rcd/rcbo

 

[baldsparkies]

If a pme system had a reading that high, I would say further investigation would be neccassary to establish if there is an underlying problem with the dno's supply.
So inform them accordingly.
An RCD would sort the problem, but what if the integrity of the earth is degrading over time and your Ze is getting higher and higher.

 

[telectrix]

be certain of your reading, then contact DNO. just fitting a RCD is like putting a plaster on a severed leg. treat the cause, not the symptom.

 

[MarkieSparkie]

Make sure you have a high current Ze reading (not no-trip), if the out of range reading confirmed, isolate the installation (you should have done this before measuring Ze of course) if you haven't already, phone the DNO immediately, this will be logged as a priority event and they will send a linesman to you (in my area within 1 hour, amazing in a rural county without motorways) to investigate and fix fault. Your TN-C-S installation is connected to a chain of protective multiple earth points (PME) and your high reading indicates a latent fault in this chain. Remember your property will amongst a number in the area connected to this network and thus potentially at risk, hence the priority for the DNO act.

 

[Engineer54]

mark80,

Is this just a theory question, or an actual situation that you have encountered today??

 

[malcolmsanford]

Hi,
Just a quick question i was thinking about today. Say if your measured ze was over the maximum value. Say for arguments sake a tncs was 0.5ohms.
Would

1) It be fine leaving it as this value aslong as all circuits fall within maximum Zs
or
2) Install an earth rod and make the installation TT, as with the ze being over the maximum value it can not be classed as a stable earth???

Aint got the regs to hand to check if theres anything in on this, and just wondered what peoples view on this would be

All sound advice about contacting the DNO and making sure you have the correct reading. But to be honest a 0.15 overeading is not going to make the DNO loose sleep over this as they know that our MFT are not that accurate, but they will check it saying that.

Oh and if the Zs is within range why are we fitting all these RCD/RCBOs

 

[mark80]

Cheers guys! The tncs was just an example. It was just a general wondering I had that's all, and just thought id ask to see what people would do I they came across it.

 

[baldsparkies]

Also worth remembering that with the nuetral and protective conductors sharing a common return path in this way. A load no load check on the voltage supply would also be worth while. It may well be that its not just the Ze thats going to be affected.

 

[mechelec]

Max Ze on a TNCS is 0.35 ohms with a 100A cutout fuse fitted. Max Ze can be higher if an 80A or 60A are fitted.

 

[johnboy6083]

on my 2391 i was told its 0.35 (TNC-S) 0.8 (TNS), but they didnt mention cutout values.

 

[ezzzekiel]

not sure on that statement my understanding is max Ze values are in place regardless of what size suppliers fuse is, the stated values would give a minimum fault current that that can be safely handled.

 

[morph]

Max Ze on a TNCS is 0.35 ohms with a 100A cutout fuse fitted. Max Ze can be higher if an 80A or 60A are fitted.

are you sure about that ?

 

[Engineer54]

There is no relation to max Ze TNC-S of 0.35ohms to the cut out fuse values... It is considered to remain the same value regardless!!