Ze

[MiAmigo]

I did a pre-work inspection at a flat prior to starting a full re-wire. The property is served by a TNS system and the Ze measured 2.7 ohms. I contacted the client's supplier (EDF) who sent out an engineer, confirmed the Ze was too high, but said there was nothing he could do. I contacted the supplier again today who said that they would "look into it again".
Surely this is a matter of life and death and in such circumstances the problem should be rectified pronto. I have always assumed that the supplier was responsible for the installation upto the meter including the maintainance of a within spec external earth fault loop impedence but what EDF appear to be saying is that as the supply to this flat is derived from a "riser" it becomes the electrician's responsibility.

Anyone else had any similar experiences?

 

[malcolmsanford]

Is the meter in the flat or in a communial area?. If the meter is in the flat and there are a set of tails from the head in say the ground floor then that would be the suppliers responsibilty someone like BG or N-Power to deal with the tails.

Where did you take your Ze from, the origin of the installation ? What was the Zdb/Zs at the flat CU.

If the DNO have accepted responsibility of supplying an earth for the installaion as a whole then they do indeed have a duty to rectify this. Normally with this type of problem of a high Ze the DNO would fit a temp RCD as additional protection while they sort out the problem, I assume they have not done this.

 

[MiAmigo]

Thanks Malcolm,

The Ze was measured at the origin of the installation, after the meter, at the main swich supply side. It is a very old installation- dating from the 30's The existing main earthing connection to the MET is not 16mm- it looks like 4mm at best. It is difficult to see where this is connected to the incoming TNS shroud as this is all behind the board. The whole installation is in serious need of updating.

EDF eventually called the client again today and suggested that whilst waiting for them to upgrade to PME (could take a month) the electrician (me) should use an earth stake and effectively convert the installation to a TT system. If the original main earth was left in place then at 2.7 ohm this would then be within spec for what would then be a TT system but I am uneasy about where this places me personally with regard to The Regulations and possible infringement.

You mention above that suppliers usually instal an RCD as a stop-gap until they have upgraded the installation. I'm wondering whether it is best to proceed along this route as the planned upgrade of CU incorporates split level RCD protection. I could temporarily run all the circuits off the RCD protected bank. Would that keep my "rear end" in the clear?

 

[malcolmsanford]

If your going to do a board change and full rewire in the near future then I can see no problems as a temp measure of fitting a single front end RCD to the installation. In all the instances I have known the DNO normally fit a 100mA one.

The only problem I can see is you may find that you'll spend more time, sorting out the existing installation to keep an RCD energised if it is that old and in that poor a condition, but I think it is something you are going to have to do for safety.

It may be time to sit down with your client and talk over the alternatives. If they are looking at a rewire I would go down the road of now fitting a temp supply for them. The evenings getting longer, so I would fit a small 2 way board, perhaps a 3 way if they need cooking, with a 30mA RCD and string some temp supplies around the house so they can have power and light and do the rewire.

Once you have completed the rewire you can TT the installation as a Temp and advise the DNO of this. I would advise the client though not to let this slide, as IMO a PME system is a better alternative to TT.

 

[MiAmigo]

Thanks for your help and responses. I have now installed an earth rod and my Kewtech KT64 gives an earth loop impedence of 8.6 ohms - so well within that required for a TT system. The new cu also has a 30ma RCD so I feel that the instalation is safe and meets the requirements of the regs.

One small question. I've now started using "FormFill" for my certificates and on its Domestic Electrical Instalation Certificate it requires numbers for both Ze and also for Electrode resistance Ra. I assume that because it is asking for both then my measured Ze is not the same as the Ra. This is the first time I've come up against a TT system so I'm not sure what is required for Ra and how to measure it.

Any advice gratefully received.

 

[IQ Electrical]

In effect, it's the same thing, Ra being the sum of the resistances of the electrode and earthing conductor.

You can enter N/A in the Ze box on Formfill if you wish.

How are you finding Formfill? We've used the full version for a good few years now and it serves us well.

 

[tony mc]

Im useing the free castline formfill for domestic and its fine.

It sometimes changes values on the schdule of test results for no apparent reason (RCD trip times and BS numbers for the OPD and ICN rating) but as long as you check it before locking and printing the original its good.

 

[MiAmigo]

Thanks IQ.
So in effect on the Formfill sheet Ze is only applicable to a supply where the Distributor provides an earth and Ra only applies when the earth is provided by an earth electrode.

I've only started using Formfill today. It is very usefull in as much as you can store information about your test instruments (Make, Model, Serial number and calibration date) and recall them for each job without laboriously writing everything out long hand. Like all new systems it will take me a while to completely get the hang of it but first impressions are that it is very usefull. It did, however, take me a while to figure out how to recall a stored file. I thought I hadn't saved it for a nerve jangling moment !

If anyone else is interested it's available as a free download from here:

Castline Systems - FormFill Level P

mia

 

[tony mc]

Just out of interest how many rods and what size did you install as thats a good Ra.

 

[MiAmigo]

One rod, 4' long, 5/8'' diameter. It went in sweet as a nut after a long wet period and into what is probably clay underneath (there are numerous disused clay pit brick works in the area). I must admit it was a lower Ra than I was expecting.