TT or TN-C advise Please

[KLV74]

Hi this is my first post and would really appreciate some advise regarding my electrical service head. Is it TNC or TT supply ?

I have an incoming cable from underground which has a fabric wrap type insulation. Out of the ISCO service head I have a red live and black neutral (no earth connection) going to my meter then 100Ma RCD and then onto an old consumer board (pop out MCB’S).

The only Earth I have is via my lead water pipe. I have checked Earth loop with Megger MT1553 and get a value of 4.5 Ohms at the board with earth disconnected (meter attached to earth from lead pipe and lincomming live) and when connected testing via socket connection 8.5 Ohms with earth reconnected.

Many Thanks K

 

[telectrix]

sounds like a TT using the water pipe. that wants sorting ASAP. either try for a PME from DNO, or whack a good sized rod in. an up to date CU iwould be good, but if it's TT keep the 100mA RCD as well as the 30mA ones in the new CU.

 

[KLV74]

Thanks but if a TT shouldnt this be ok as my Ra is less than 200 Ohms ?

 

[telectrix]

it's OK at the moment, but what if the water supplier fits plastic pipe?

 

[Engineer54]

Thanks but if a TT shouldnt this be ok as my Ra is less than 200 Ohms ?

Not if your Ra is based solely on a water service pipe which is basically NOT allowed. Any incoming service pipe should only be a bonding connection to the installations MET and Not your installations earthing electrode!!!!

 

[monkeyelectric]

Not if your Ra is based solely on a water service pipe which is basically NOT allowed. Any incoming service pipe should only be a bonding connection to the installations MET and Not your installations earthing electrode!!!!

Its allowed on a private water supply but not public or shared system.
cheers Tel

 

[telectrix]

other way round. it is allowed on a private water suppy as long as permission from the owner is obtained.

 

[monkeyelectric]

other way round. it is allowed on a private water suppy as long as permission from the owner is obtained.

Yeah thats what i meant but i see it reads different how i've written it, private allowed, shared or public a definate no no

 

[Engineer54]

Unless its on a private water supply, then i thought it was allowed

I doubt very much if you would gain permission from any private water suppliers to allow their pipework to be used as your electrical earth electrode!!. Now days, private means your own water supply from an Athenian well or the like!! lol!!

 

[KLV74]

Thanks, appreciate this, I will bang a spike in. Would there be any value in asking my supplier if my cable sheaf can used as an earth.

 

[monkeyelectric]

I doubt very much if you would gain permission from any private water suppliers to allow their pipework to be used as your electrical earth electrode!!. Now days, private means your own water supply from an Athenian well or the like!! lol!!

Yeah thats what i always deemed a private supply to mean was your own well on your own property and anything else was deemed public

 

[craig1000]

Doubt they will say yeah. All like to get you to out a rod in or charge for them to come and PME nowadays in my experiences so far! Had one similar a year or so ago and it was the earth rod for me as they wanted to charge to install PME, customer wanted cheapest option so earth rod it was! ;-)

 

[bertt]

Hi,
About TT or TN-C
Could you do me a favour please.
Take a picture with your meter for me, especially I want to see the cable body and the end of that. If you don't mind.
Thank you.
Bertt.
London

 

[Engineer54]

Thanks, appreciate this, I will bang a spike in. Would there be any value in asking my supplier if my cable sheaf can used as an earth.

No, install a deep roded TT system, at least 2 X 5/8'' rods coupled together, to give you depth which will give you both stability of the electrode and a decent Ra value!! Which when connected to the MET with your water bonding cable, should give you a very acceptable TT system Zs value!! Let us know what Ra value you obtain and your Zs values when you've finished. BTW your Ra value will also improve over time as the ground consolidates around the earth rods. Take another reading off the rod installation during the summer and at the end of the year!! lol!!

 

[Engineer54]

Doubt they will say yeah. All like to get you to out a rod in or charge for them to come and PME nowadays in my experiences so far! Had one similar a year or so ago and it was the earth rod for me as they wanted to charge to install PME, customer wanted cheapest option so earth rod it was! ;-)

The PME connection charge would have made MUCH more sense than your earth rod charge!!