High Ze

[geezza]

Could someone please advise me what the standard procedure would be if you were to get a high Ze reading of 1.48 Ohms, it is a TN-S installation

 

[mechelec]

Get in touch with your DNO and they will send someone out within 4 hours as it a safety issue.
Will be fixed FOC.

 

[malcolmsanford]

You would have to make sure it is a TN-S and not a TT. Also make sure you meter is ok, nothing quite as embaressing as when the DNO turn up the Ze is 0.25 or something.

If it is defiantely a TN-S then the DNO have taken the responisibilty of providing an Earth and you need to call them on their emergency number, normally on a bill and tell them what you have found, they will be round toot sweet and sort it out.

If the good old BS 951 clamp as been used on the sheath then they may just do a better job and braize a proper connection. Chances are they will just tighten it, and may even arrange to change the supply to a TNC-S

Whatever you do though don't try and repair whatever is there, it's the DNO's material. As your Ze is high what are your Zs results.

 

[geezza]

My Zs is 1.76 on a 32A MCB

 

[malcolmsanford]

It won't disconnect in time , think it should be about 1.16 off top of the head, definately a call to the DNO now.

 

[telectrix]

to slightly diverse, here's one that has me concerned. on a TT system, say if your Ze is 19.something the your Zs is going to be Ze+R1+R2 i.e. 19or 20 ohms. How do you equate this with a max. permissable Zs of say 1.16

 

[geezza]

I'm going to recheck the reading and if I still get a high reading I will contact DNO

 

[sjm]

to slightly diverse, here's one that has me concerned. on a TT system, say if your Ze is 19.something the your Zs is going to be Ze+R1+R2 i.e. 19or 20 ohms. How do you equate this with a max. permissable Zs of say 1.16

This is why all TT systems require additional protection with RCDs in order to meet the disconnection times.

 

[telectrix]

so, if i read you correctly, the disconnection time is reliant on the RCD, not the MCB.

 

[telectrix]

in other words, the Zs and Ze readings arew irrevelant from a practical point, just record them on the test sheet?

 

[sparkylad]

No T. the ze and zs are certainly not irrelevant.
You still need to verify that you have an acceptable and stable reading at the Earth electrode, For a 30ma rcd you could mathematically have an Ra of 1667 ohms, however a max of between 100 ohms and 200 ohms is deemed the acceptable stable value. you are additionally still required to measure the zs of each circuit to indicate integrity of the Line/cpc fault loop continuity.


sparkylad

 

[sjm]

For TT systems I usually put n/a for max Zs allowable under BS7671 and then record the tested value in the relevant box.
Ze and Zs are still relevant though. sparkylad is spot on.

 

[Risteard]

This is why all TT systems require additional protection with RCDs in order to meet the disconnection times.

Theoretically speaking they don't necessarily require RCD protection to meet disconnection times if disconnection times can be achieved otherwise. However it is unlikely that this will be the case.

Additional protection is a different concept however.

 

[brucelee]

to slightly diverse, here's one that has me concerned. on a TT system, say if your Ze is 19.something the your Zs is going to be Ze+R1+R2 i.e. 19or 20 ohms. How do you equate this with a max. permissable Zs of say 1.16

by protecting with a 30mA rcd which will allow a theoretical value of 1667ohms but may be unstable over 200ohms fot tt

 

[Guest123]

For TT systems I usually put n/a for max Zs allowable under BS7671

IMO that is the incorrect thing to do. Depending on what rating of RCD you have there IS a maximum permitted Zs that must not be exceeded in order to achieve disconnection times.