tt or tns?

[lionelmcc]

Hi
I was looking at a job the other day which had a recent minor works cert which had the arrangement down as TT.
On further invesigation I discovered that the 16mm earth cable did indeed leave the building but went up the wall outside and was connected into a 3 wire overhead supply.
I am presuming this is TNS or am I barking up the wrong wall?

L

 

[jimcallear]

I'd say that without checking how its connected up there you cant be sure as to if it's a tns or tnc-s.

 

[Spudnik]

I would contact the DNO.

Sounds a bit odd to me.

 

[Guest123]

Agreed. I aint never seen one of our earthing cables going up the wall and connecting into the OHL. Very weird

 

[johnnyb]

I have never seen that done either, something not quite right there, the DNO don,t run earth wires to there overheads, they usually are pme at the cut out, they are around here anyway.

 

[lionelmcc]

Ok, I'll investigate further and keep you all posted

I have never seen that done either, something not quite right there, the DNO don,t run earth wires to there overheads, they usually are pme at the cut out, they are around here anyway.

I will contact the suppliers on Monday, however I have just been back to the site and followed the poles back a few hundred yards, until it disappeared into wood.
The house is an old gate lodge for to a larger house, which I am also looking at doing some work on.
The big house would appear to be supplied by the same method and, on looking inside would at first glance, seem to have a PME arrangement although there is no PME label at the main fuse or meter.
also , when driving home I did notice lots of poles ( all over the place) with three wires

I feel Like I am on a different planet

 

[Des 56]

It may be a third wire earth This would probably be a overhead TT supply Not seen much nowadays

 

[johnnyb]

Hia mate, the only time i have seen anything of the like is when the DNO has brought there own earth off the pole to an earth rod or plate near their pole, but never back into a dwelling or such like, interesting though what u have said.

 

[Guest123]

Hey.

Des seems to have the most logical opinion. But then that 3rd wire could also form part of a TN-S system, depends where and how the other end is connected

Very difficult to tell really.

 

[lionelmcc]

i followed the poles back for about 400 yds until they crossed a stream and on into the woods
then walked on to the house which has the same arrangement with the wires coming back accross from a different part of the woods
it's difficult walking country

 

[lionelmcc]

Ok the plot thickens

I took a reading fo Ze; 0.46. Would suggest PME?

when I phoned he DNO (Scottish Power) I was told that they did not have that information.
I was then condescendingly told by some jumped up receptionist, that, "Any electrician worth his salt would be able to tell"
At this point I am glad that I am not the only person on this forum who is confused

Comments please

L

 

[mrloy99]

Hi
I was looking at a job the other day which had a recent minor works cert which had the arrangement down as TT.
On further invesigation I discovered that the 16mm earth cable did indeed leave the building but went up the wall outside and was connected into a 3 wire overhead supply.
I am presuming this is TNS or am I barking up the wrong wall?


L

sounds pretty obviously a TN-S.Separate earth throughout.Your ZE reading is also too high for TN-cs.
The suppliers have moved away from using these systems nowadays due to economical reasons and safety(cost of extra cable,and possibility of gypos stealing it for scrap) But as it is a pretty old installation by the sound of it TN-S sounds favourate.

 

[Spudnik]

Agree with the above.

PME max Ze 0.35 and TNS is 0.8, so there must be a path somewhere!

How did you test the Ze?

 

[lionelmcc]

Hi jason

I disconnected the bonding and tested between the incoming earth and the henly from the L tail

What about the attitude of the DNO?

 

[Spudnik]

Hi jason

I disconnected the bonding and tested between the incoming earth and the henly from the L tail

What about the attitude of the DNO?

Im afraid its the luck of the draw who you get when phoning them.

You could try phoning back and asking to speak to someone more in the know. They are there.

Remember, when you call them, the people that answer the phone do not have a lot of technical knowledge.

I struggle when i phone to book a PME enquiry.

Is there any way you can get us some pics?