Electric shock off live steeltech shed!!

[An Buachaillín Bán]

Hello all,

Looking for help here as I've come across a strange call-out today. Bare with me here:

Got a call from a customer saying they had received an electric shock from their steeltech shed, not once but twice.

To give a bit of a background, there are two sheds in their back garden. One concrete and one steeltech shed. Both approx 8m from the house.

The concrete is the original shed, being installed over 30 years ago. It has a small sub-board inside, with a 10amp mcb for lighting circuit (two flourescents) and 20amp mcb for a socket circuit(two double sockets-rarely used). There's a seperate earth rod driven outside the shed too.
Sub-board is fed from the house, an old Baco 63A E.L.C.B, coming off a 38A mcb. Cable to shed is a 6sq SWA cable, buried through the garden and out to the shed. Nothing too untoward so far.

About 8 years ago, they had a steeltech shed put in, about 2-3m from the existing shed. It is at a higher elevation than the first shed as its closer to the house. An electrician took a 2.5 SWA cable from the 20A MCB in concrete shed and ran it into the new shed. But he ran it into a JB, and from there, split the feed- one direct to a metal clad double socket and the other to feed a switch and thus the light.

Here's where I got worried. Everything was working fine for years, no issues. But in the last few days, when the owner touched the steel frame or metal lock on the door he'd get a shock. When I got there I had only time to do a quick inspection for the obvious ones, screw through cable etc. I isolated power and opened up the socket, light, switch and JB. Nothing caught by a screw or anything. I did notice the steel frame wasn't bonded anywhere though. I left socket plate open and turned on power. Went from L terminal at socket against the steel frame of shed and got 230v reading. I disconnected the SWA feeding the shed at JB and still got 230v from L to steel frame. So that ruled out any issues at the accessories in the new shed.

Obviously when I killed power to the 20A mcb it killed any power in the steel shed. I checked the SWA cable where I could and all seemed to be fine. It was buried for about 2m but owner told me it had even been buried in 2 inch pipe. No digging or any civil works lately either. Nothing tripped when he got the shock. I haven't performed any testing yet, I just isolated the ELCB and 38A breaker in the house and even put down mcbs in shed.

Before I go back, was wondering if you had any suggestions at to what it could be?

Thanks all.

 

[pc1966]

An (VO)ELCB won't trip on earth faults outside of its own CPC system (e.g. a TT earthed shed, person touching L and standing on grass, etc, etc) and even then, tripping on its own CPC is not that reliable.

Really the supply to those buildings should be on a RCD type device so the current imbalance trips it, not what it thinks the CPC is relative to some earth rod (that may or may not be still good enough).

Start by IR checking the new shed's system, repeat on the old shed's system. But really you must do something about any ELCB as they can't be trusted to provide adequate protection.

 

[westward10]

You need to do basic checks. Is there earth continuity to the electrical installation in the Steeltech shed because a breakdown of this could result in a potential difference to the shed if it is earthed.

 

[An Buachaillín Bán]

An (VO)ELCB won't trip on earth faults outside of its own CPC system (e.g. a TT earthed shed, person touching L and standing on grass, etc, etc) and even then, tripping on its own CPC is not that reliable.

Really the supply to those buildings should be on a RCD type device so the current imbalance trips it, not what it thinks the CPC is relative to some earth rod (that may or may not be still good enough).

Start by IR checking the new shed's system, repeat on the old shed's system. But really you must do something about any ELCB as they can't be trusted to provide adequate protection.

Thanks for the reply. Yes, I'm intending on replacing the ELCB and MCB with a new MCB and putting in a new sub board with RCBOs. Will be carrying out checks tomorrow

 

[An Buachaillín Bán]

You need to do basic checks. Is there earth continuity to the electrical installation in the Steeltech shed because a breakdown of this could result in a potential difference to the shed if it is earthed.

Thanks for the reply. I did a continuity check on the cable across L+CPC. No evidence of bonding, but shed shared electrode with original shed.What I don't understand is that when I had the cable feeding the steeltech shed disconnected from any load, I still got 230v across L and the steel frame.

 

[timhoward]

Is the main house TT (as suggested by the ELCB) or has the earthing changed?

 

[westward10]

Thanks for the reply. I did a continuity check on the cable across L+CPC. No evidence of bonding, but shed shared electrode with original shed.What I don't understand is that when I had the cable feeding the steeltech shed disconnected from any load, I still got 230v across L and the steel frame.

If from #1 you proved earth continuity by measuring voltage between two points then this is not an effective means of proving continuity.

 

[pc1966]

Thanks for the reply. Yes, I'm intending on replacing the ELCB and MCB with a new MCB and putting in a new sub board with RCBOs. Will be carrying out checks tomorrow

First you have to establish that the main house is on a TN style earth so a MCB feeding the SWA to the sub-board is actually capable of disconnection on an earth fault.

If the house has an ELCB it almost certainly was TT, that might have been changed now (as I understand the ROI has basically mandated TN-C-S supply) but that is something you have to check via Zs measurement or similar at the feed point to the SWA cable.

Does the house have any RCD protection on any sockets? If not then you really should be discussing an upgrade that replaces the ELCB with a modern RCBO (or at least dual RCD) board in the house for that reason.

 

[timhoward]

What I don't understand is that when I had the cable feeding the steeltech shed disconnected from any load, I still got 230v across L and the steel frame.

My question about earthing type was related to this part. This needs understanding, as a matter of urgency.

 

[davesparks]

Put one probe of your voltage tester on the steel shed and the other probe into the soil, see what voltage you get.

Has the shed been set up as a TT system or is it just an earth electrode connected to it in addition to a connection back to the MET?

 

[davesparks]

An (VO)ELCB won't trip on earth faults outside of its own CPC system (e.g. a TT earthed shed, person touching L and standing on grass, etc, etc) and even then, tripping on its own CPC is not that reliable.

Are we sure it is a VOELCB and not a current operated ELCB?

A quick Google search for Baco ELCB seems to show that they are still making and selling RCDs under the description of ELCB.

 

[davesparks]

What I don't understand is that when I had the cable feeding the steeltech shed disconnected from any load, I still got 230v across L and the steel frame.

The steel shed is presumably in contact with the ground so it will be connected to the electrical earth because of this, so it is reasonable to exoect to see a voltage between live and the steel shed.

 

[pc1966]

Are we sure it is a VOELCB and not a current operated ELCB?

Indeed we are not. Some photos, etc, would have helped a lot!

 

[timhoward]

The steel shed is presumably in contact with the ground so it will be connected to the electrical earth because of this, so it is reasonable to exoect to see a voltage between live and the steel shed.

It’s all a bit confused as he originally says

Went from L terminal at socket against the steel frame of shed and got 230v reading. I disconnected the SWA feeding the shed at JB and still got 230v from L to steel frame. So that ruled out any issues at the accessories in the new shed.

I read that as with supply removed he was picking up 230v. My concern is/was that as the house owner was getting shocks off the steel, it may be a broken PEN situation. Unlikely but not ruled out yet.