Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> TN-what? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1162140746 Message started by londonman on Oct 29^th, 2006, 4:52pm

Title: TN-what?
Post by londonman on Oct 29^th, 2006, 4:52pm

More out of curiosity than anything else I'm trying to work out what type of electricity connection we have. Reading this page on wikipedia http://en.wikipedia.org/wiki/TN-S none seem close to what we have as far as I can see.

We are fed from a transformer on a pole that converts the 11kV to mains voltage. It's hard to see but I'm guessing that there is a connection to earth from the transformer (ie I'm guessing a great spike in the ground). From the transformer to the house is overhead armoured cable but no connection to the outer sheath of the armoured cable at the house end. When it reaches the house, the neutral is bonded outside to an earth wire down to an earth spike.

Inside the house, when the neutral reaches the company fuse, there is another (very thin 4mm?) black wire that is connected from it to the earth bar in the CU.

Does this make sense?

If so, what type of system do we have?

Is the 4mm earth wire adequate?

The CU is a split load with a time delayed 100mA RCD as part of the main on/off breaker and then a separate 30mA RCD feeding the usual suspects.

What are the implications (if any) of running power to an outside workshop? I'm thinking about whether an earth spike is needed at the workshop end etc.

I'm just trying to understand a bit more before I get an electrician in so I can have a sensible discussion with him.

Title: Re: TN-what?
Post by The_Trician on Oct 29^th, 2006, 5:00pm

Your supply is TT - Overhead.

This is why there's a time-delayed RCD in the consumer unit acting as the main switch.

'TT'

Title: Re: TN-what?
Post by Lectrician on Oct 29^th, 2006, 7:01pm

Lets have some photos ;)

Title: Re: TN-what?
Post by The_Trician on Oct 29^th, 2006, 10:14pm

Good idea -

Or -

Where the LV system is overhead, including where the service cable runs underground from a pole outside the property, the earthing system is likely to be TT.

Where the LV system is entirely underground, the earthing system is likely to be TNS.

In both cases, it is possible for the REC to convert the system to TNC-S (PME) by the addition of additional earths at certain places throughout the LV system. They should place a sticker adjacent to the cutout stating that the supply is PME.

For the final word, contact the local REC. This information is one of the things they have to confirm under the ESQCR2002 regs, which they all work to.

TT

Title: Re: TN-what?
Post by RabbitRabbit on Oct 30^th, 2006, 2:10pm

A picture wopuld indeed help. It sounds to me like its TN-C-S or PME. BUT you mention a small conductor (csa) connected to the neutral as an earth.

As most PME'ed TT systems a relatively new I am surprised it is so small.

As TT says there should be a label at the incomer head stating it is PME, if indeed it is. Before contacting your REC/DNO regarding the possibility of PME'ing the system check at the pole carrying the cables, if it is possible to PME your installation then there should be a label on that pole stating it is PME.

If PME you get get a sparky to check the Ze for you and if its PME'ed then you can do away with ther RCD as the main isolator, if you so choose, but there are implications to consider.