High Ze on a TT system - help for a newbie please.....

[The General]

I was called out to replace a couple of pendant fittings and install a fused spur for an extra socket into a cupboard.
Installation done, all dead tests were fine, but live tests came up with a Ze of 806 ohms. Zs was similar figure.
30mA RCD (within timings for x1 and x5) fitted before entering an old Wylex board with rewireable fuses if that makes any odds.

I know that max Ze for a TT is 1667 but that anything above 200 "may be unstable", but just wanted to check a couple of things.

Firstly,
As I already installed the additional socket, what do I do if the client says they don't want to install extra earth rods?
(they're the sort of people who say "it's been there for 30yrs and no-one's died yet...")
If the regs say that the maximum is 1667, but >200 may be unstable does that mean that I can walk away knowing it's no worse than it was before I started?
Document on Minor Works to say that I've advised client earthing needs improving, or do I HAVE to do something about it?
(and, yes, I know, morally I should do something about it...)


Secondly,
having never had to install additional rods before, and looking at a few other posts on here I'm suddenly panicing about how deep these things need to go?!
Only did one before with another spark and it was about a metre long, but seen a few posts about putting rods in 5-10m (or more) deep!
Any advice/guidance or top-tips would be gratefully received.
I'm thinking with a figure of 806 ohms already that the ground is pretty dry :-(


p.s. and, yes, I'm kicking myself for not doing a quick Zs check at a socket when I came out to do the quote before I started work on it ...... lesson learnt!

many thanks in advance for your help.

 

[rich.250]

Basically the lower the better is the best approach. Can't say I'm one for this 200 ohm stability figure plucked out the air. Lol.

I would be tempted myself to put another rod in/ possibly hit the old one in a bit. Just for piece of mind for the little time it would take.

Bet you are kicking yourself now

 

[Strima]

Give it a few more days and take another reading, bet it's gone through the roof then. Did you check the earthing arrangement, corrosion, continuity or breaks etc?

 

[The General]

I would be tempted myself to put another rod in/ possibly hit the old one in a bit. Just for piece of mind for the little time it would take.

Bet you are kicking yourself now

Yup.... lol.

So, let's say an extra rod brings it down to 400ohms (as an example) - I could record that on my Minor Works along with advice of "possible instability of earthing arrangements"?!?
Is it only ever acceptable to have a figure of <200 or is it 'technically' accceptable with a higher one?

 

[The General]

Give it a few more days and take another reading, bet it's gone through the roof then. Did you check the earthing arrangement, corrosion, continuity or breaks etc?

To be honest I was running out of time by the point I was doing live tests, so I just took the Ze measurement (swore a lot!) and then booked another date as I didn't have any rods on the wagon with me. Didn't get a chance to do a really thorough inspection of it other than to check there was a rod, and it did actually have a G/Y wire attached....

 

[Richard Burns]

806 ohms sounds high if the rod is installed in the ground correctly. Check the rod location just in case it is installed in conduit or something!!. I have had a 31 ohm result from a rod inside an outbuilding.
Is the connection outside protected? often not so it could be just corrosion of the joint.
Definitely worth doing a continuity test from where it joins the MET back to the rod to check for problems there.
If you have a rod with the possibility of extending it then that is your easiest option.

I think (but do not know) that legally you could leave it since the 200ohms figure is just advice and you are unlikely to go above 1667 if you start at 800. However that said get a decent Ra value with good protected connections and long rods.

 

[rich.250]

Yes you could technically leave it, inform the customer etc.
But I couldn't do it,
I would do as Richard burns says above. At the end of the day it's the most important cable in the property!!

 

[Engineer54]

I know that max Ze for a TT is 1667 but that anything above 200 "may be unstable", but just wanted to check a couple of things.

If it's a single 1m rod of any size it's going to be ''unstable'' whether it has an Ra of 800 ohms or 200 ohms!! And you can take that to the bank!!

No-one on here to my knowledge, has stated that you drive a 10m rod in the ground for a domestic. i doubt very much if you could drive a rod in to that depth!!

As for the rest of your post, ...Your the electrician, so it's up to you how hard you push your clientsto get the fundamentals of this installation right!!

As a minimum on a domestic, you should be looking to provide your customer with at least 2 X 5/8'' coupled rods. (with a flush to ground earth pit to protect the cable to rod connection. That will give you at the very least ''Stability''.....

Do you think an up front RCD to an old Wylex 3036 fuse board is suitable for a modern TT system installation?? Didn't think so!! lol!!! All you can do is advise them accordingly, it's there decision at the end of the day. As the old saying goes, ...you can lead a horse to water, ...but!! lol!!

 

[Inteificio]

Preparing to get shot down for this but here goes.

There is no legal requirement to get Ze that low, it is completely unnecessary as long as the RCD works.

I would inform the customer what you have found and leave it in their court.

Option 1: 1 RCD - insist that they test it frequently and that if they don't there is a chance of failure. - legal minimum.
Option 2: 2 RCD different brands, covers your arse more, odds of simultaneous failure slim
Option 3: See if you can get a PME set up,
Option 4: Quote for earth rod

It is the customers property, it is not your responsibility to fix everything to your standards, just keep to the law and keep it safe. It is their money, give them the choice of what you do.

 

[Des 56]

It is the customers property, it is not your responsibility to fix everything to your standards, just keep to the law and keep it safe.

M'Lord,I refer you to regulation 132.16

The last word in the paragraph is adequate,now in good old Clint Eastwood mode
Well punk,do you feel lucky

 

[The General]

Many thanks to all for your thoughts/advice.

Sounds like it's a case of revisiting property with wander lead to check continuity to MET, having a good fiddle with connection at the existing rod to check for corrosion, and sinking a new rod (in a pit) and see where it gets us!


watch this space if I'm still struggling....

thanks again!

 

[telectrix]

maybe a different location for the rod might help, and get 2 coupled rods. hopefully you can get them both in before you hit rock (or engineer54 in china)

 

[Strima]

Decent SDS with rotary stop, get that bitch in there!!! :lol:

 

[gazzamikes]

Chuck the lovely cup of the tea the customer has made using full fat milk and tea leaves! Be quick to take a reading before it starts to dry up

 

[trev]

Chuck the lovely cup of the tea the customer has made using full fat milk and tea leaves! Be quick to take a reading before it starts to dry up

Like .3 of a litre of fluid is going to make any difference?

 

[Inteificio]

Like .3 of a litre of fluid is going to make any difference?

Any tea served in a mug smaller than a pint is not worthy of an electrician.

 

[telectrix]

the only thing that should be served as a pint is beer.

 

[gazzamikes]

I was testing earth rod couple weeks ago at 463 ohms and rcd wouldn't trip couldn't work it out, rammed another rod in and it then started tripping albeit just inside the limit. But on my assessment he said 1660 is the max 200 is only an advisory

 

[Welchyboy]

These readings seem v high, maybe it's the soil type where you are, I'm in south east and even with a meter long skinny rod, the highest reading I've ever got is 86 ohms, it averages around 20/30 ohms, the best ever being 13 ohms, I usually try to install a 2 meter rod but have to cut it short sometimes as It will not go any deeper, I pilot it with a meter long 12mm sds bit first then use a paving mallet, been looking at the dewalt rod driver lately though!

 

[Engineer54]

These readings seem v high, maybe it's the soil type where you are, I'm in south east and even with a meter long skinny rod, the highest reading I've ever got is 86 ohms, it averages around 20/30 ohms, the best ever being 13 ohms, I usually try to install a 2 meter rod but have to cut it short sometimes as It will not go any deeper, I pilot it with a meter long 12mm sds bit first then use a paving mallet, been looking at the dewalt rod driver lately though!

I personally wouldn't go to the expense of a dedicated rod driver. A half decent SDS drill with a rotary off position will be more than adequate for the needs driving up to 3 X coupled 5/8'' 1m rods!! You can make your own, or purchase an SDS tool to fit the driving nut. Apart from anything else, using an SDS unit will give you an easier drive and far better results. Using a club/sledge hammer will have the rod springing and flexing all over the place, and ending up with the top of the rod sitting in a hole 10X the diameter of the rod ...lol!!

As i have stated many times now, the use of a single 1m rod of any diameter will in 99% of times leave you with an unstable rod. A rod needs depth to achieve stability....
You'll stand a far better chance of going down the full depth of 2X coupled if you stay well away from building external walls (at least 1 or 2m), ...as that's where you'll find all the old building rubble the original builder buried...lol!!!

 

[Welchyboy]

I personally wouldn't go to the expense of a dedicated rod driver. A half decent SDS drill with a rotary off position will be more than adequate for the needs driving up to 3 X coupled 5/8'' 1m rods!! You can make your own, or purchase an SDS tool to fit the driving nut. Apart from anything else, using an SDS unit will give you an easier drive and far better results. Using a club/sledge hammer will have the rod springing and flexing all over the place, and ending up with the top of the rod sitting in a hole 10X the diameter of the rod ...lol!!

As i have stated many times now, the use of a single 1m rod of any diameter will in 99% of times leave you with an unstable rod. A rod needs depth to achieve stability....
You'll stand a far better chance of going down the full depth of 2X coupled if you stay well away from building external walls (at least 1 or 2m), ...as that's where you'll find all the old building rubble the original builder buried...lol!!!

Will give that a try!!