Routing of submains to TT installation - been picked up on PIR and not sure??

[cornburn]

Hey all,

A recent PIR has thrown up an issue and I'm really not sure what to do about it.

Submain from the main building electrics room (PME) routed underground and enters the external building (TT) and is made off into plastic up in the loft. Protection for the submain is by way of 60A switched fuse. After being made off into the plastic box, the inner cores are then run inside some metal trunking down a wall and into a metal VIGI unit for RCD protection for the building. It has been picked up that all the cables prior to the RCD need to be encased in plastic rather than metal.

So, short of ripping out the very expensive metal merlin VIGI unit and replacing with a very expensive plastic vigi unit and extending and re-routing the armour cores, does anyone have any other suggestions?

I don't really understand the logic with this, as with no one ever fiddling around inside the VIGI unit the chances of one of the cables wearing through and shorting through to the metal are nil. Would replacing them with double insulated tails be another option? It's been picked up on the last few PIRs and every time it doesn't get changed as I guess everyone who's come along has thought it's OTT and couldn't be bothered!

Another option I guess would be adding time delay RCD protection at the supply end but again we're looking at high costs and getting rid of the asbestos insulated fuses and switch.

What would you do? Cheers

 

[malcolmsanford]

a Vigi is not a containment system, a vigi is an RCD, well in fact they are an accessory that attaches to an MCB that makes them an RCBO in other words. so using metal and then plastic in terms of a vigi is throwing me a little. Unless a vigi you are talking about is something totally different, perhaps I have it wrong

I would imagine that there could be 2 areas for concern here.

1. Separation of the earthing systems PME/TT
2. Metal containment in the building that as the TT system

As for 1 well the SWA ( I assume) being terminated into a plastic box should insure you have system separation, as long as you have not extended the PME into the TT system

As for 2 well yes you need to really protect your TT containemnt/DB if they are metal with RCD protection

With a TT system you will need to use a plastic CU/containment since this minimizes the dangers of a L/E fault occurring inside the Containment /CU before the RCD (and hence never being cleared due to the high earth fault loop impedance

To be honest I would think the PIR/EICR picked up the second reason. Double insulated tails will of course be better, to protect the cable, but it is really the high Zdb/Zs you will get on a TT, and the danger of metal that calls for either, change the metal containment or use as you say a S type RCD for the tails.

Out of curiosity what is the DB made from.....also metalclad?

 

[cornburn]

a Vigi is not a containment system, a vigi is an RCD, well in fact they are an accessory that attaches to an MCB that makes them an RCBO in other words. so using metal and then plastic in terms of a vigi is throwing me a little. Unless a vigi you are talking about is something totally different, perhaps I have it wrong

I would imagine that there could be 2 areas for concern here.

1. Separation of the earthing systems PME/TT
2. Metal containment in the building that as the TT system

As for 1 well the SWA ( I assume) being terminated into a plastic box should insure you have system separation, as long as you have not extended the PME into the TT system

As for 2 well yes you need to really protect your TT containemnt/DB if they are metal with RCD protection

With a TT system you will need to use a plastic CU/containment since this minimizes the dangers of a L/E fault occurring inside the Containment /CU before the RCD (and hence never being cleared due to the high earth fault loop impedance

To be honest I would think the PIR/EICR picked up the second reason. Double insulated tails will of course be better, to protect the cable, but it is really the high Zdb/Zs you will get on a TT, and the danger of metal that calls for either, change the metal containment or use as you say a S type RCD for the tails.

Out of curiosity what is the DB made from.....also metalclad?

Cheers for that

The earthing is separated fine, the problem that has been picked up is that the armour cores are running through metal trunking, then through holes in the metal case of the vigi. This is located next to the main CU, which is also metal but obviously RCD protected by this point.

The paperwork the PIR company sent in complains that the cables are passing through 2 holes in the metal vigi unit, and that they should be in plastic trunking and enclosure until after the unit. There's physically no room to put plastic trunking in and reroute without extending the incoming cables, hence why I'm wondering if there's anything else can be done to meet the requirement.

As far as I can see the problem is just that the cables are passing through metal holes before they are RCD protected.

 

[Engineer54]

I'm just wondering why they didn't run the SWA directly to the location where the supply is to be utilised, saved on all that unnecessary trunking metal or plastic!!! The PME could have been isolated before entering the metal enclosure housing the Vigi unit!!!

By the way, if the tails are entering the metal housing for the Vigi unit through separate holes, then this is a definite No No, and should be corrected before you do anything else!!!

 

[malcolmsanford]

If it is just a matter of the cables entering the metal containment through different holes can you not either

1. Slot between the holes using a jig saw

2. Actually slot the thing out entirely and then use the plastic sleeve to protect the cables around the slot

 

[cornburn]

I'm just wondering why they didn't run the SWA directly to the location where the supply is to be utilised, saved on all that unnecessary trunking metal or plastic!!! The PME could have been isolated before entering the metal enclosure housing the Vigi unit!!!

By the way, if the tails are entering the metal housing for the Vigi unit through separate holes, then this is a definite No No, and should be corrected before you do anything else!!!

This was my thinking, would have been so much less messing around to just use an insulated gland onto the vigi unit.

I'll have to see about slotting the holes to keep IP ratings close enough, but I still think the issue they're picking up on is that the vigi unit is metal. Does anyone know specifically if this is a no no? If so I'll have to extend the cores through some plastic trunking and replace the vigi case with a plastic equivalent. PITA!

 

[Knobhead]

With regard to the slot between the holes, I used to fill them with epoxy resin. That way the IP rating is maintained.

 

[Engineer54]

This was my thinking, would have been so much less messing around to just use an insulated gland onto the vigi unit.

I'll have to see about slotting the holes to keep IP ratings close enough, but I still think the issue they're picking up on is that the vigi unit is metal. Does anyone know specifically if this is a no no? If so I'll have to extend the cores through some plastic trunking and replace the vigi case with a plastic equivalent. PITA!

As far as i'm aware, there is no hard and fast rule that enclosures have to be Plastic on a TT system more of a reccommendation. As far as i'm conserned, so long as any entry points have been suitably bushed or otherwise protected, then the tails/cables are going to be protected!! I certainly wouldn't be pulling out all that metal trunking and enclosures just for the sake of it...lol!!

 

[mechelec]

Unless I am reading this wrong it sounds as if someone is getting confused. Reg 531.4.1 (sorry 17th edition not ammended) says that an RCD must be at the origin unless the parts of the installation between the origin and the device complies with the requirements of class 2 insulation. In your installation the submain cable is supplied from a PME earthing system, therefore it should have been designed to comply with ADOS requirements for PME and be protected against earth faults by a suitably low earth fault impedance . I imagine the SWA is is terminated into a plastic enclosure to insulate the supplied building from the PME protected armour. There is no reason the cores cannot be run in metal trunking. The circuits supplied from the submain are then on a TT system and are protected by the VIGI unit for earth fault protection and therefore metal or class one enclosures can be used after that point.