Very Hi ZS Reading on TT system? any advice

[srl-8]

Hi guys,

i have changed a fuse board today in a outbuilding/large shed (it has a kitchen and shower and lounge etc.. a )

It is a TT system with earth rod.

My ZE reading was 187 ohms - now the regs state I am ok upto 200, so I assumed this was ok.

All the R1 + R2 and insulation resistance came back normal.

When I carried out the ZS tests, the reading were very high, for example:

Shower circuit on a 40a MCB - 2.6 omhs !
Kitchen Ring on a 32a mcb - 2.7 omhs !

same for lounge and bedroom rings.

I have checked to regs and these figures are over the maximum allowed.


Any advice ???

 

[Simlec]

This is going to be a very long thread me thinks! :rolleyes4:

 

[One_of_them]

if your R1+R2 readings came back as satisfactory then that circuit impedance should be OK. Therefore you need to start looking at other factors relating to this issue i.e your Ze!! Are you earthing conductors connected correctly and secure and in good condition?

Get another rod sank in and see if this is any better to lower your Ze

 

[Specialist]

Discrimination ? i.e is the RCD feeding it in the House a time delayed type.

 

[IQ Electrical]

ZE was 187

the system is protected by a 30ma RCD from the main house, and the new board is also protected by a 30 ma RCD.

??

Ok then your maximum allowed Zs reading for each 30mA protected circuit is 1667 Ohms, problem solved in 31 posts.

Your RCD at the supply end should ideally be an 'S' type time delayed to achieve discrimination with the RCD at the remote end but I wouldn't loose sleep over that one.

 

[Richard]

ZE reading 187ohm

I can determine ZS by enquiry, ZE +R1 +R2 = ZS

However, i tried to test and measure ZS at the end of the circuit and got these high figures of 2.6 2.5 etc etc...

again, any ideas on how to solve this issue???

Thanks

ok, first it's not an issue.

When you tested Ze, You disconnected the earthing conductor which went to the earth rod I imagine.

As previosuly mentioned, you may well have bonding in place to the Water service???

Try testing with the earthing conductor re-connected, and you will probibly get a reading of somewhere around 2 ohms. As previsouly stated by telerix, by connecting the water pipe you've effectivly got a much better earth rod!

With regards to the values in the Regs - Being a TT system, because your Ze is not garenteed stable, circuits must be protected via RCD (in your case 30ma), which from what you say they already are.

 

[mechelec]

ZE was 187

the system is protected by a 30ma RCD from the main house, and the new board is also protected by a 30 ma RCD.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

I was too slow in replying to specialist (too slow, specialist beat me) sorry to cause confusion

 

[IQ Electrical]

I'm busy writing an FAQ on TT systems for a certain organisation, judging by some of the questions (and answers) on this thread, I need to write faster!

 

[GLENNSPARK]

and if you are going to start driving more rods in...then go for depth every time ...."depth is the key with TT".....

 

[GLENNSPARK]

ZE was 187

the system is protected by a 30ma RCD from the main house, and the new board is also protected by a 30 ma RCD.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

too slow, specialist beat me

i can feel discrimination entering the fray here!!..lol....

 

[Geordie Spark]

and if you are going to start driving more rods in...then go for depth every time ...."depth is the key with TT".....

Now THAT could have been posted by our good Friend E54 !!

 

[Engineer54]

I refuse to get involved!! lol!!! ....But please, when refering to TT systems it's ''Ra'' NOT Ze....

 

[biggles]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

 

[Specialist]

ZE was 187

the system is protected by a 30ma RCD from the main house, and the new board is also protected by a 30 ma RCD.

would that be a 30mA RCD protecting the sub main and another 30mA RCD protecting the final circuits in the outbuilding i.e. 2 RCD's in series

too slow, specialist beat me

Hey: I've got to beat someone at posting at least once a year:lol:

 

[Geordie Spark]

I refuse to get involved!! lol!!! ....But please, when refering to TT systems it's ''Ra'' NOT Ze....

Now that's a shame ..... I enjoy reading your posts.

 

[IQ Electrical]

sorry if i read that wrong mechelec but why would 2 rcd`s in series slow the disconnection time

It wouldn't, in practice, both devices would trip under fault conditions because there would be no discrimination without the upstream device being an 'S' type time delayed.