Shock off the earth bar

[La Poste]

Greetings.
I was at a friends house today who has a TT system earth rod.
I am helping with the wiring.
All bonding is in place he has a 17th edition dual RCD board.
I was wiring in a standalone RCD unit for his garden shed and got a shock off of the earth bar.
I am concerned because he wants to put all brass switches on his lights.
I don't know why I got this shock but if his earth bar is live then anything connected to it will be live also.
None of the RCD's have tripped and I cannot understand why I got this more than finger tingling shock.
I understand that appliances can have earth leakage that can accumulate at the earth bar in the board but I consider this a potentially dangerous situation.
If there is earth leakage then why no RCD tripping?
Any comments would be appreciated.
Thanks.

 

[Deleted member 9648]

Do a Ze test on the earthing lead disconnected from the MET...reading should be less than 200 ohms but readings under 1667 ohms will still provide adequate fault protection if the RCD's are OK....did you test the RCD's for correct operation and tripping times?

 

[Darkwood]

The problem with tt systems is if your in a street and the neighbours house isnt as good an earth as yours then its possible for a fault to travel between premises and use the earth rod in the house your in, it may be due to variation in ground resistance and may have travelled up you to the earth bar, no rcd's will trip as the power isnt coming from the supply cable in your installation, id check the p.d. between the ground or wall and the earth bar.
Ive had this before where when i bonded the piping the gas engineer got a shock and after days of trying to figure it out we pulled the cutout fuse and guess what the voltage was still present, it turned out an old supply joint in the road had corroded losing earthing to several houses, when we rodded and bonded (due to high ELI) a fault from 3 doors up used my nice new earthing as its path, we had local clark of works, niceic, and safety guy saying i was wrong but when i withdrew the cutout (for safety reasons) and proofed the fault was external the then they backed off and got the electric board in.
This isnt too common and may not bare any similarity to your issue but its worth a thought.

 

[La Poste]

Do a Ze test on the earthing lead disconnected from the MET...reading should be less than 200 ohms but readings under 1667 ohms will still provide adequate fault protection if the RCD's are OK....did you test the RCD's for correct operation and tripping times?

Earth rod is about 200 ohms but with bonding in place it goes down to 17 ohms that's why I am concerned, the bonding could be live.
Earth rod is being replaced this week.
All RCD's trip in about 17ms at 150ma and 35ms at 30ma although light circuits have not been tested.
In fact I am going to get in my car now and have a look with my voltmeter.
Tell you later what I frind.
Cheers.

 

[Deleted member 9648]

Good luck....sounds an odd one.

 

[La Poste]

Just got back.
Tested everything and zero potential across all earths and rod, bonding, between neutral and earth, I touched the earth bar and nothing happened but I was thinking.
Before I connected up this armoured cable that travels down to the bottom of the garden I IR'ed it at 1000v just to see.
Is it possible to charge a cable and then touch it later and get a jolt? The shock I got felt like a low current shock but high voltage. It didn't feel like mains it felt odd like a spark and a tingly feeling in my fingers. Mains voltage usually grabs hold of my whole arm but this was different.

 

[telectrix]

could it have been static? i have had instances where office staff had had shocks from metal ( earthed ) fixtures through static

 

[La Poste]

Well if it was it was a big old ball of it.

 

[Darkwood]

what worries me is you say mains voltage usually grabs your arm like its a regular thing

 

[olly w]

Sounds to me like you have charged up the armoured cable going down the garden and got the shock off that.
Don't forget that an IR test is DC and this effectively turns the cable into a capacitor, holding the charge. (until you discharge it)

 

[La Poste]

Sounds to me like you have charged up the armoured cable going down the garden and got the shock off that.
Don't forget that an IR test is DC and this effectively turns the cable into a capacitor, holding the charge. (until you discharge it)

I'm hoping it's that because otherwise it's an intermittent fault being brought in from outside somewhere.

what worries me is you say mains voltage usually grabs your arm like its a regular thing

I seem to be having quite a good run at the moment.
Is there a discharge proceedure or a period of time to wait before the cable is safe?
I'm guessing it would be best to short the cable out after the tests have been completed.
This is a new one on me. Next time I might charge the cable up with 1000v and then take a volt reading across it.
One more thing, I might see if I can get a voltage alarm of some sort, a buzzer that sounds if the measured voltage from rod to earth bar becomes reversed IE current flows from the rod to the board, there must be something out there. I could then tell at what times of day this occurance occurs.
Cheers.