Discuss 80amp DP Isolator and a 100amp Main Switch do you need both! in the Electrical Forum area at ElectriciansForums.net

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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...
 
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S

stuart569

Is it an Mccb or a switch fuse? The chances of it ever blowing are slim to none so really isnt doing any harm and is very handy for board changes etc ... like your doing
 
D

Deleted member 26818

Just remember, that the DP isolator will be your main switch on the paperwork.
 
P

paddyscouse

i wouldent trust the dp isoater, on any instalation i rarther be safe and pull the fuse
 
  • Thread Starter Thread Starter
  • #9
Is it an Mccb or a switch fuse?
The current DP isolator is a Wylex CAT WEM 80/2 100Ma Trip 80A Load 240v 50Hz. The new board is a QFS-X16M with a 100A Main Switch (two way) so that would shut off the neutral and the live conductors.

telectrix said:
useful to isolate CU without cutting seals.
Yep that's part of the argument for keeping it in, any future changes to the board could be isolated from the seperate DP isolator.

spinlondon said:
Just remember, that the DP isolator will be your main switch on the paperwork.
Good point, thanks.
 
Just remember, that the DP isolator will be your main switch on the paperwork.
The DP isolator does not have to become your main switch on paperwork if there is one inside the CU. 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, as this is the one the customer has access to in every case.
 
S

stuart569

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

SKY

I too would pull the fuse and keep it in my pocket until I have finished the work.
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.

OK I work mainly on commercial properties where there is a chance of someone doing the above so always try to isolate in a fail safe way.
 
D

Deleted member 26818

The DP isolator does not have to become your main switch on paperwork if there is one inside the CU. 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, as this is the one the customer has access to in every case.
I think ELECSA need to re-read the Regulations.
Specifically: 537.1.4 "A main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation.....".
 
O

oldtimer

I may be picking this up wrong but the op is concerned that he has a 80A switch feeding a 100A switch in a CU now what he does not mention is the size of the cut out fuse. So here is my tuppance worth dont pull the fuse switch off the 80A isolator test to confirm that the outgoing cables are dead then disconnect them and now you are good to go . Fit the new CU do your tests get your tails ready and again test the 80A isolator then connect your tails ensuring you have the CU main switch off then once you are happy switch on the 80A isolator and go through the proving sequance at the CU main switch and because you have done all your tests do a final check on ALL of the CUs connections and now you can switch it on

Or am I missing something


I forgot to add this happens all the time in flats that have 60/80A fuses but when you fit a new CU or any CU they always have a 100A rated main switch plus thats the last thing to worry about as people are fitting 11kw showers in flats
 
I think ELECSA need to re-read the Regulations.
Specifically: 537.1.4 "A main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation.....".
I don't see how this excludes the use of the DP isolator at the CU as suitable for being used on forms tho spin.
 
D

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.
 
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".
 
K

kellogs64

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

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.
 
S

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

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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

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Arms
Esteemed
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.
 
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

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

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?
 
  • Thread Starter Thread Starter
  • #27
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.

Guitarist said:
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:
 
O

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
 
  • Thread Starter Thread Starter
  • #31
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
No problem, just wanted to see what people thought about having two pieces of isolating equipment between the CU and the Service Head. Personally, I think it's a little old school to have a further isolator outside the consumer unit. I'm all for keeping it neat and tidy within the consumer unit. Then all the end user has to understand is that if they have a power problem then all the on/off switches are in one place, easily accesible.

I was just waying up the advantages/disadvantages of keeping the old isolator in place. One of the disadvantages of removing it would be losing an 100ma RCD on the main switch. Without an RCD the main switch wont trip which I think is a bit annoying because in the new RCBO board the home owner would have to go along turning circuit after circuit back on, without an overall control. Not sure I rate that, seeing as thats what the owner had before, although RCBO's will be a significant step forward instead of the current rewirable situation.

One possibility would be to buy a different main switch with 100ma RCD, pull the fuse and get rid of the old wylex isolator allowing the owner to keep control over all the circuits in the event of a problem, although the RCBO's are very safe so I'm splitting hairs...
 
H

Heisenberg

Is it considered acceptable to have any sort of RCCB as a main switch? Surely even a 100mA one might allow for nuisance tripping :confused5: albeit not as much as a 30mA device.
 
G

Guest55

If theres a 100ma stand-alone rcd i'm instantly thinking TT unless i'm missing something ???
Has the OP confirmed supply earthing configuration ??
Lots of other basic errors being made in this thread sadly.
 

telectrix

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Arms
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the whole point of the up front isolator is to enable safe isolation of the CU without resorting to cutting seals.
 
  • Thread Starter Thread Starter
  • #36
If theres a 100ma stand-alone rcd i'm instantly thinking TT unless i'm missing something ???
Has the OP confirmed supply earthing configuration ??
Lots of other basic errors being made in this thread sadly.
It is a TT with a high 386 Ohm ze reading as well...
 
G

Guest55

It is a TT with a high 386 Ohm ze reading as well...
This info may of been helpful at the beginning of the thread ??? :-/
Anyway if youre gonna fit a new dual rcd CU i'd be replacing the 100ma rcd with a DP switch.
And the rod Ra could do with improving as well.
 
  • Thread Starter Thread Starter
  • #38
This info may of been helpful at the beginning of the thread ??? :-/
How's that then? The original post was about getting rid of an electricity isolator switch and replacing it with a main switch inside a new board.

Anyway if youre gonna fit a new dual rcd CU i'd be replacing the 100ma rcd with a DP switch.
The new board will have a dp 100A main switch and all circuits will be RCBO'd.

And the rod Ra could do with improving as well.
I'm going to have to fit another rod to try and bring that reading down hopefully.
 

telectrix

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Arms
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start by replacing the rod with screwed together 5/8" rods. go at least 3 ft. from the house wall to avoid bob the builders rubble.
 

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