First TT test and inspection

[Spoon242]

Guys,

I have just been working through a TT system round an old work colleagues home as it has given me the opportunity to look into TT's.

He has a the incoming supply through a 100mA switch which carries on in to a 10 way Wylex board in the house with BS 3871 breakers and its own main switch.

I took my Ze by isolating the house removing the main earth cable from the MET and using my Fluke 1653, on no trip Ze, took the reading. Result was 530 ohms. From looking at the regs I can see this is high, as a 100mA should be 500 ohms, ideally 200 ohms, Table 41.5.

Do I need to remove his earth spike and renew it, or add an extra spike as a course of action.

Secondly, Zs readings for the house circuits are taken in the normal way I'm assuming, but of course I'm getting readings like, for example,

Cooker 508 ohms
Induction hob 501 ohms.

Im thinking these are high, please slap me in to shape on this, and ten worse what tables do I look the max Zs up in for TT's

Bloody 2391 was always geared up for TN system Grr!

Thanks in advance for your help as always.

 

[telectrix]

first, replace the earth rod. use jointed rods 5/8", and go deep. then retake your Ze. on a TT system, ALL circuits must have 30mA RCD protection.

edit: with 30mARCD/s, your max. Zs is 1667 ohms.

 

[Spoon242]

So Tele,

If I recommend changing the 100mA switch to a 30mA switch, this will be his cheapest option. rather than a Cu change and I will look at the earth rod change, can you possibly link me 5/8" earth rods so I can see what is need required.

 

[telectrix]

only problem with upfront RCD is a fault on any 1 circuit will take out the whole installation. as for rods, you can get 5/8" rods with threaded ends and couplers , so you drive the 1st one in using an impact drill, screw on the next one etc. don't damage the threads though. the 3/8" twigs most wholesalers call earth rods ain't worth bothering with.

 

[Spoon242]

Hmm, will hire an impact drill, or is it the same as rotation stop?

 

[Engineer54]

As Tel stated discard the existing rod, probably one of those useless 3/8'' rods anyway!! Find a suitable spot at least a metre or two away from house exterior walls (where you are most likely to find all the old building rubble) to drive in at least two 5/8'' coupled rods (don't forget to buy a percussion head bit that fits into the coupler, ...keep forgetting the correct name for them. ..lol!!) You can make the driving job a lot easier if you can weld/braze a correct sized old socket onto a cut down old SDS drill bit.

Don't forget to provide a flush to ground proprietary earth pit, that will protect the rod connection and provide a visual location for the earth rod.

As for relying on the numpty recommendation 200 ohm and an RCD, check out the other threads here on this issue... lol!!!

 

[Deleted member 26818]

The requirements to provide 30mA RCD protection are no different for TT systems than for TN systems.
If there are circuits that would not require 30mA RCD protection in a TN system, then 100mA or even 300mA RCD protection for those circuits would be acceptable.
Circuits that have no RCD protection would require some other method of fault protection, such as double or reinforced insulation.
For instance the supply conductors and the meter tails.

 

[Engineer54]

Forgot to give you the link to earthing rods as requested...
Here goes... Furse Earthing and Lightning Protection and Electronic Systems Protection by Thomas & Betts

 

[Spoon242]

Hey guys,

I have finally completed the upgrade to the earth rod and now have a reading of 140.8 ohms. Thanks to the peeps that helped.
If I understand I can now use the Ze from TN systems to apply to my readings, is that right?

 

[Guest123]

.
If I understand I can now use the Ze from TN systems to apply to my readings, is that right?

A little confused by this??

Can you explain your thoughts a little more??

 

[Spoon242]

A little confused by this??

Can you explain your thoughts a little more??

Lenny, forget my crazy post. I thought I had read this somewhere in the OSG.

But of course its total rubbish as 1667ohms is the key to meeting Zs, which I have easily met.

 

[Des 56]

Hey guys,

I have finally completed the upgrade to the earth rod and now have a reading of 140.8 ohms. Thanks to the peeps that helped.
If I understand I can now use the Ze from TN systems to apply to my readings, is that right?

?
You use the value for TT that corresponds to the rating of the rcd

Too late with that post, you knew the answer all the time,naughty man :stooge_moe:

 

[Guest123]

as 1667ohms is the key to meeting Zs, which I have easily met.

For a 30mA RCD yes.