80amp DP Isolator and a 100amp Main Switch do you need both!

[rustynails]

I've seen various posts about this but wanted to start a new one, nothing like repitition. I am installing a new CU and it currently has an 80amp DP isolator between the service head and two split old wylex rewirable boards. I will be installing a 14 way board which comes with an 100amp Main Switch and every circuit will be protected by an RCBO. Seeing as it's illegal to tamper with the service fuse... I guess it is advisable to leave the DP isolator in place, it just seems unecessary.

Anyone have any thoughts on this...

 

[Deleted member 26818]

The forms require the details of the main switch.
The switch in the CU cannot be the main switch, if there is another switch between it and the origin of the installation.

 

[Guitarist]

We will have to agree to disagree on this one mate. I have to agree with Elecsa here, as I would consider either one the main switch for the "installation".

 

[kellogs64]

what about the diffrence between them,shouldnt you change it for descrepancey

 

[Deleted member 26818]

We will have to agree to disagree on this one mate. I have to agree with Elecsa here, as I would consider either one the main switch for the "installation".

Two main switches, I suppose next they'll have two main earth terminals.

 

[Snapester]

i wouldent trust the dp isoater, on any instalation i rarther be safe and pull the fuse

Why not just leave connection to main switch last then you dont have to worry about it ever being energised without you know on the board change! Me myself i always work in a nice set of marrigolds!

 

[telectrix]

i think i agree with spin on this. the "origin" of the installation is " the position at which electrical energy is delivered to an electrical installation". to my mind that point is the DNO fuse, therefore the main switch is the closest point of isolation to the fuse.

 

[imago]

It's not something I'll get too concerned about, but I'm with spin on this. Even if you ignore the regs (what a nice thought!) a main switch would be the single switch which controls the largest amount of circuits after the head/meter. So it has to be the first isolation point after the head/meter.

When you consider that there can be up to 3 metres of cable between the isolator and the CU which the CU switch does not cover then the switch in the CU can only isolate the installation partially even if it is the largest part.

That said, from what I can see of paperwork relating to installations 90% is worthless or missing. That which does exist is rarely used or seen by anyone other than the spark who wrote it, and maybe another spark working on the system at a later date. So you could probably note the transformer up the road as the main switch and no one would say anything.

 

[Guitarist]

Two main switches, I suppose next they'll have two main earth terminals.

There often are 2 main earth terminals, 1 block and 1 in the CU. Thing is, we tend to designate 1 and use that for connections (although I often see both used). As long as it is stated on the paperwork which one is the main isolator, then we have a main isolator for our paperwork. Remember, this is only for our form, and as long as they both work and comply, then my personal belief is that the one which could be used by the customer is the most important one.
Anyway, I'm out of this now before you get too upset with me.

 

[telectrix]

LOL, as long as things are clearly documented. the BS no is the same for either SW, just the rating might be different.

 

[drew35]

So if the person who posted that he removes the main fuse for "safety", has the main fuse in his pocket whilst "on the bog", is happy to leave an exposed un-fused live terminal. Because someone might switch an isolator back on? (Which is supposed to be lock off with a sign in place)

Do these main fuses get any kind of seal put back at the end of these jobs? Or is it ok that anybody could pull it without the use of a tool, and then stick their finger on the fore mentioned un-fused live?

I can only guess that it must be much harder in some areas of the Country to dial a number and ask for the DNO to meet you their to remove the fuse and put an isolator in place. Or even before you arrive? Hard to imagine?

 

[Guitarist]

That's why "dummy" fuses are always used Drew

 

[rustynails]

So its a main switch rcd. What earthing system does the house have?

No, the new Main Switch inside the CU is not an RCD just a 2 Pole 100A main switch. The current isolator is a DP Wylex CAT WEM 80/2 100Ma Trip 80A Load 240v 50Hz, this has an RCD.

I had a similar situation a few weeks ago and checked with Elecsa, and the opinion was that either can be used on the form, just as long as it conforms with regs. Personally, I use the CU isolator

If this is true, then I too think I would prefer to use the new Main Switch as the primary supply overcurrent device.

If I did the isolator and I could not lock it off correctly it would be easy for someone to come past and turn the isolator on while I was on the bog.

This is true, I don't think I can lock off the current isolator off.

now what he does not mention is the size of the cut out fuse

The service head fuse is 60 amps.

The switch in the CU cannot be the main switch, if there is another switch between it and the origin of the installation.

This was also my take on the regulations too. Primary switch being the first piece of isolating equipment between the CU and service head.

:rolleyes2:

 

[Guitarist]

Remember rusty, a switch is not an overcurrent device.

 

[rustynails]

Remember rusty, a switch is not an overcurrent device.

That's true my mistake. The DNO fuse is the primary supply overcurrent protective device.

 

[oldtimer]

I dont know what the problem is here you have a 60A DNO fuse a Wylex 100mA RCD 80A rated switch that supplies a consumer unit that has a 100A rated switch inside it so what am I missing here ?

As I mentioned in a earlier post a 100A rated main CU switch is standard in but the DNO supply cna be 60A-80A or 100A either way the switch is fit for purpose I would be more concerned if there was a 100A DNO fuse and 10 way CU with a 60A rated main switch