TN-S TO TN-C-S

[Traineeboy]

I went out today with one of the lads who was asked to change a customer to tn-c-s from tn-s. The DNO had just put in a new service head and this one had an active PME. The chap just removed the earthing conductor of the sheath of the cable and put it into the service head to move the customers earthing over to TN-C-S. I raise a point with him that even through the customer has a PME Supply the sheathed TN-S should also be connected as its a extraneous conductive part. He said its OK as not PME. 

Am I right in thinking that even tho the supply is now TN-C-S the TN-S Cable sheath should still be earth as its a extraneous conductive part ? Just like a water or gas pipe

 

[Sharpend]

Who asked him to change it? 

 

[binky]

so in short the new service head is designed for TN-C-S, so they just ditched the earth? 

 

[Traineeboy]

Who asked him to change it? 

The DNO did the work of a new service head and said that they left the TN-S connected but the customer would need to get an electrician to connect a new earthing conductor to the service head 

 

[binky]

The DNO did the work of a new service head and said that they left the TN-S connected but the customer would need to get an electrician to connect a new earthing conductor to the service head 

OK so it's one of those service heads with a little cover where the earth terminal is that you can remove without tampering with seals? If so, remove cover and see if there is a link bar to the neutral or not, or the earth is terminated separately from the old sheath earth.

 

[Traineeboy]

OK so it's one of those service heads with a little cover where the earth terminal is that you can remove without tampering with seals? If so, remove cover and see if there is a link bar to the neutral or not, or the earth is terminated separately from the old sheath earth.

yes it’s one of those. When the earth is tested via the sheath he got 0.27 and from the earth bar in the service head he got 0.15 so it’s clearly a separate earth from the sheath. My thoughts are that yes a earthing conductor should be connected in the service head ( to make PME) but the old TN-S earth should also be left as the sheath needs to be bonded as it’s technically a extraneous conductive part. Would I be right in my thinking ? 

 

[Fleeting]

A picture may make things clearer.

 

[binky]

yes it’s one of those. When the earth is tested via the sheath he got 0.27 and from the earth bar in the service head he got 0.15 so it’s clearly a separate earth from the sheath. My thoughts are that yes a earthing conductor should be connected in the service head ( to make PME) but the old TN-S earth should also be left as the sheath needs to be bonded as it’s technically a extraneous conductive part. Would I be right in my thinking ? 

I would tend to agree, but, DNO rules are different to ours. It could certainly be argued that it's introducing a different earth potential to the building. However, as it stops at the service head, is there much chance of being in contact with it, and a.n.other item under fault conditions? 

 

[roys]

Nothing to add to this thread except to say that fair play to @Traineeboy he asks some good quality logical questions.

 

[kerching]

I must agree that across all of @Traineeboy questions ALL of them are relevant AND he is the only one that tries to come up with his own answer first and asks for clarification ...he will go far

 

[Traineeboy]

Thanks chaps I appreciate your insight. Always will bring my knowledge to the table and weigh it up against those who have experience. Just like clarification which I’m very grateful you all take the time to chip into my knowledge. I normally get laughed at work. Thanks again. 

 

[binky]

Thanks chaps I appreciate your insight. Always will bring my knowledge to the table and weigh it up against those who have experience. Just like clarification which I’m very grateful you all take the time to chip into my knowledge. I normally get laughed at work. Thanks again. 

the guys training you do seem a little lacking  :slap

 

[Traineeboy]

Yeah my placement it rubbish but sticking with it for now. 

 

[Sharpend]

At least you’ll learn what not to do! 

 

[kerching]

normally get laughed at work. Thanks again. 

They laugh because they don't KNOW!

 

[Murdoch]

Yeah my placement it rubbish but sticking with it for now. 

doing this job means asking questions , however daft they can seem to be, and that is a continual situation we are in

i like the way you ask your questions and offer up your thoughts too