Metal clad distribution boards and TT

[Sidewinder]

OK, unphased, been busy again tonight and popping in and out of the forum.

I promised a response so here we go.

You will have to participate in this though else it will make no sense!

I am doing it this way, not to make a fool of you, but so anyone who is learning can follow the debate and hopefully learn.

When you have a TT supply from the DNO, what do you get from them?

What is the maximum acceptable limit "in the trade" for the RA in a TT install?

Please respond and we'll move on.

Sorry it is so short, I'm very busy at the moment!

Breakdowns and behind on other work with a client because they are always adding extras!

 

[unphased]

Morning.

As I understand it, the DNO do not install the rod. They expect the rod to be installed by the customer. Recommended high Ra is 100ohms. Max recommended is 200ohms but is considered unstable.

Next. O)

 

[Sidewinder]

OK, unphased you are correct, the electrical contractor has to install the rod, and your RA figures are correct, (ish) above 200 ohms is considered unstable, but that is academic in this situation, as will become clear.

Please work with me on this one.

OK the DNO provide the live conductors, that is a phase and a neutral.

So we run these into a metal enclosure which is at the origin.

This is connected to the means of earthing for the installation.

In our case the rod, with an RA of 50 ohms measured by the Ze method as everyone understands this and has the kit if they are registered.

There is no gas in the premises and the water is a polymer incomer, thus no main bonding required as the heating is all electric.

No electric shower either.

The main fuse is a BS88 100A.

In the event of a dead short between the line conductor and the body of the metal enclosure located at the origin, what will be the fault current?

(Simple ohms Law will be fine.)

How long will it take this fault current to clear the fault by "blowing" the 100A main fuse?

What will be the voltage sitting on the body of this metal enclosure when this fault current is flowing?

Cheers.

 

[unphased]

Fault Current 230/50 = 4.6A

Insufficient current to blow the 100A main fuse.

230V sitting on the metal enclosure.

 

[Sidewinder]

Hi unphased, I did not expect such a quick response!

Obviously you are correct.

So, would you still be happy putting a metal enclosure at the origin of a TT supply when you consider that my scenario is possible?

 

[unphased]

In the situation you describe it would be very dangerous and clearly unsuitable, So an RCD would be used at the origin....then a metal enclosure could be used. Hence that explains the reg.

 

[Sidewinder]

No an RCD would still NOT disconnect the fault I describe, the only protection would be the DNO fuse, hence my point, which has again been missed.

HOW can an RCD disconnect a fault UPSTREAM of its location?

 

[steptoe]

Im either reading that post wrong, or somebody is?

---------- Post Auto-Merged at 21:39 ---------- Previous post was made at 21:38 ----------

ah, definition of origin may be the problem for me, sorry.

 

[unphased]

No an RCD would still NOT disconnect the fault I describe, the only protection would be the DNO fuse, hence my point, which has again been missed.HOW can an RCD disconnect a fault UPSTREAM of its location?

The origin of an installation is the position at which electrical energy is delivered. So how can it be upsteam? In my mind I am imagining the main fuse is the origin. Therefore by introducing an RCD in the meter tails (after the meter) the tails entering the metal enclosure are protected fromearth faults by the RCD.

 

[park1]

The origin of an installation is the position at which electrical energy is delivered. So how can it be upsteam? In my mind I am imagining the main fuse is the origin. Therefore by introducing an RCD in the meter tails (after the meter) the tails entering the metal enclosure are protected fromearth faults by the RCD.

That's fine. You either place a RCD at the origin or you ensure it comply's with double or reinforced insulation.

 

[Sidewinder]

That's fine. You either place a RCD at the origin or you ensure it comply's with double or reinforced insulation.

NOT if the metal enclosure is around the RCD, you have to put he RCD "in" something, this is my point.

That FIRST enclosure CANNOT be metallic.

Thus the first enclosure in a TT install has to be insulating.

After that & the RCD then you are fine to use a metal enclosure.

 

[park1]

NOT if the metal enclosure is around the RCD, you have to put he RCD "in" something, this is my point.That FIRST enclosure CANNOT be metallic.

Thus the first enclosure in a TT install has to be insulating.

After that & the RCD then you are fine to use a metal enclosure.

Disagree there, you just need to ensure the cable's are protected upon entry, although i can see why GRP is preferable.

 

[Sidewinder]

Disagree there, you just need to ensure the cable's are protected upon entry, although i can see why GRP is preferable.

$park,

This is where it all started.

If you refer to the regs that have been quoted you will see what I am getting at.

Oh & it does not have to be GRP, very few enclosures these days are TBH.

All it has to be is insulating.

It does not matter how the tails are protected unless both them and the enclosure meet the requirements of the relevant regs.

 

[Phoenix]

I am told that some manufacturers provide some kind of plastic insulating shield kit for when a metal board is used on a TT system with an RCD incommer, I've never seen one on a domestic consumer unit

What I have seen, in commericial where the supply has been TT and the main RCD has been a 60947-2 device with earth leakage shunt trip fitted (think merlin gerin virgi units... but this was the ABB equivelent) was even though the enclosure was plastic, the breaker was mounted to a metal plate inside. Now to protect the incomming tails from contacting this, it was overlayed with a sheet of clear perspex with larger dimensions so that it over hung the edges of the matal plate, so that there was no way that the non RCD feed could contact the metal. This was the design of the panel builder who was resposible for building the ABB unit into a GRP enclosure with a few other bits. I made sure that on site PVC trunking was used to contain the tails prior to the RCD unit.

After attdenting to a fault where TT earthed metal work had come into contact with a non RCD live conductor, I'm expecially carefull about this kind of thing!!

 

[park1]

$park,This is where it all started.

If you refer to the regs that have been quoted you will see what I am getting at.

Oh & it does not have to be GRP, very few enclosures these days are TBH.

All it has to be is insulating.

It does not matter how the tails are protected unless both them and the enclosure meet the requirements of the relevant regs.

Just use insululated bushing's

 

[Sidewinder]

$park,

OK, so you could put the tails in through separate holes in the steel enclosure with say plastic compression glands?

This would ensure that they could not contact the steel body.

Would you find this acceptable?

 

[park1]

$park,OK, so you could put the tails in through separate holes in the steel enclosure with say plastic compression glands?

This would ensure that they could not contact the steel body.

Would you find this acceptable?

Not quite, they come as a kit supplied by the manufacturer,an insulating plate containing the glands, and a small trunking system/cable retainer ensuring the cable's take the shortest route and are protected throughout to the terminals of the RCD.

 

[Sidewinder]

Right, OK, I am fully aware of these "kits" not a fan, but that is personal preference, not regs.

However, you no longer have a standard metallic enclosure.

That is almost my solution anyway, you could just stuff a bit of flexible conduit over the tails inside the enclosure, that would be the equivalent.

So what is the problem with taking my first suggestion and adding a bit of flexy conduit, as long as it is all polymer?

 

[Phoenix]

If you take the tails through separate holes with stuffing glands in a ferious metal enclosure then our friend Mr. Current, first name eddy comes into play!

 

[unphased]

Hi Sidewinder. Thanks for your perseverance. I can see your point about metalic enclosures on a TT installation. All things considered, best avoided me thinks. ; \