Does a TT Earth 0.13Ohms require a 100mA incomer?

[Energetic]

Had an interesting one this week, customer assures me he's on TT, DNO days the same but i'm getting 0.13Ohms on the Ze, never seen it that low befor.

There's no 100mA RCD on the supply and it doesn't look as if there's ever been one, so with that reading i'd assumed it was a TN system... so my question is do i now need to put on it as its TT? I'm assuming i do but with a reading that low is it still required?

My Regs book is in the van elsewhere... Thanks.

 

[telectrix]

are you sure you dissed all parallel paths when measuring?

 

[dazza1981]

Had an interesting one this week, customer assures me he's on TT, DNO days the same but i'm getting 0.13Ohms on the Ze, never seen it that low befor.

There's no 100mA RCD on the supply and it doesn't look as if there's ever been one, so with that reading i'd assumed it was a TN system... so my question is do i now need to put on it as its TT? I'm assuming i do but with a reading that low is it still required?

My Regs book is in the van elsewhere... Thanks.

Also that reading can fluctuate quite a bit aswell depending on the time of year,ground conditions etc

 

[Des 56]

With regard to the actual question,do you now need to put one in?

This is a good question for a less than common situation


With your book in the van,a sentence relevant says
(ii) overcurrent protective device
An appropriate ocpd may be used for fault protection provided a suitably low value of Zs is permanently and reliably assured

They do not say a low Ra,its a low Zs

That would seem to indicate that the value includes, where it exists,the service bonds,structural steel etc

In a house the helpful bond may be a metalic water pipe that is making the reading fine,fine that is until it is then changed to plastic,not good

So back to the Ra,if it is as low as you say and the electrode is visibly apparent then it needs no Rcd,that is if the reading is under the 200 guide (only quoting not agreeing) and its reliable and the bonds are assured to stay intact
This my interpretation (rather than knowing or having had a situation that you describe)

On a domestic that would not be assured and the Rcd should be fitted,but the wording of the regs suggest it can be used to negate the need for an Rcd in other circumstances

 

[Engineer54]

Had an interesting one this week, customer assures me he's on TT, DNO days the same but i'm getting 0.13Ohms on the Ze, never seen it that low befor.

There's no 100mA RCD on the supply and it doesn't look as if there's ever been one, so with that reading i'd assumed it was a TN system... so my question is do i now need to put on it as its TT? I'm assuming i do but with a reading that low is it still required?

My Regs book is in the van elsewhere... Thanks.

With an Ra that low, treat it as you would a TN system. Certainly doesn't need any special considerations does it!! lol!!

Mind you i'm more than just a little skeptical if this is a domestic installation. If on the other hand we are talking about a unit on an industrial estate, there could well be a ground ring or ground field covering the whole estate...

 

[SPARTYKUS]

It'll not be a true Ze I'm sure. The water pipe will be connected to the neighbours supply, which will have a TNCS earthing system or TNS.

I suspect when you isolate the rod it will not give the same value!

in the above situation, the house may very well truly be TT, but with an incredibly low Zs on circuits due to the parallel path to a TN System.

thats what my moneys on!

 

[sonobit]

I agree, have a look for some commons signs like overhead supply and such like, if you cannot find the rod for the Ra then maybe like people have said its now a TN system. From my experience with DNO theyre not always right :-z

 

[Engineer54]

It'll not be a true Ze I'm sure. The water pipe will be connected to the neighbours supply, which will have a TNCS earthing system or TNS.

I suspect when you isolate the rod it will not give the same value!

in the above situation, the house may very well truly be TT, but with an incredibly low Zs on circuits due to the parallel path to a TN System.

thats what my moneys on!

True but we can only go by what the OP has posted, and if he says he conducted a Ze test, then all other parallel earths would not be part of the test circuit. He also stated the TT earthing had been confirmed by the local DNO, leading me to believe that TNC-S/PME is not available at this time in that area....

 

[tbs1983]

I recall the figure 21 ohms is the lowest a TT system can be and 200 ohms the highest. would need to check up on that though

 

[Energetic]

Thanks for all the responses guys.

You're all a bit right, i did not make much of an effort to find parallels but there's no TN at this end of the village.

It's a DIYer who thinks he knows what he's doing being advised by a mate who half knows what he's doing. There was an unspecified issue in the past and the earth is run in 35mm to two stakes, which would explain a solid reading but its still lower than i would have expected.

I suppose its been a good few months for earth resistivity, not much drying out going on but there's a stable block on a submain that we were actually working on its earth stake was 154Ohm. and all is covered at that end by RCD.

I think i'll just be putting one on to cover all bases but appreciate the help and very interesting to get others take on it.

e

 

[Deleted member 9648]

I recall the figure 21 ohms is the lowest a TT system can be and 200 ohms the highest. would need to check up on that though

Lol....best check up then.

 

[Amp David]

I recall the figure 21 ohms is the lowest a TT system can be

HaHa! The op had best nip of to his wholesaler and pick up a few boxes of resistance to pour over the electrode.

 

[dazza1981]

I recall the figure 21 ohms is the lowest a TT system can be and 200 ohms the highest. would need to check up on that though

Explain mate...

 

[SPARTYKUS]

I recall the figure 21 ohms is the lowest a TT system can be and 200 ohms the highest. would need to check up on that though

Yes, you need to check up on this subject. The figures do mean something, but in reality the resistance of an earth rod can be, in my best Catherine Tate, "whatever"

 

[Energetic]

I recall the figure 21 ohms is the lowest a TT system can be

HaHa! The op had best nip of to his wholesaler and pick up a few boxes of resistance to pour over the electrode.

.

It's buried sadly, I could saw the 35mm back to about .5mm?