Using MCB's in parallel to give local isolation

[FatBob]

This installation has a very old DNO metal fuse box with a hinged lid, which may or may not open in one piece.
The DNO's meter feeds 2 x 63A MCB's in a 2 slot CU.

Can I feed a Henley block with both line connections from the 2x63A MCB's, thus using it as a local isolation switch?
The 80-100A DNO fuse should blow before the combined MCB's operate, assuming the current is split equally, which is should be.
The Henley block can then feed a normal CU.

Bit unconventional, but trying to think out of the box whilst waiting for DNO to fit local isolation?

 

[DurhamSparky]

sorry not 100% sure what your trying to achieve with the 2x 63A MCBS...!

 

[telectrix]

no way mate. suppose you had to isolate the circuit and switched 1 MCB off. circuit would still be energised through the other 1.

 

[Guest123]

Are you saying there is an MCB in the neutral conductor????????

 

[Marvo]

An MCB and an isolator are two physically different things. Most MCB's aren't rated as isolators.

 

[FatBob]

no way mate. suppose you had to isolate the circuit and switched 1 MCB off. circuit would still be energised through the other 1.

It would be clearly labelled, and your isolation test at the CU should show if it was isolated.

 

[FatBob]

Are you saying there is an MCB in the neutral conductor????????

No, phase

 

[nikmet]

It would be clearly labelled, and your isolation test at the CU should show if it was isolated.

Bad practice at the least?

 

[FatBob]

An MCB and an isolator are two physically different things. Most MCB's aren't rated as isolators.

Providing the rating is greater than the DNO fuse. But there is a point there, an isolator will isolate phase and neutral

 

[FatBob]

Bad practice at the least?

I agree that anything unconventional is potentially creating problems for the next spark.

 

[FatBob]

sorry not 100% sure what your trying to achieve with the 2x 63A MCBS...!

Don't want to touch the DNO fuse, and don't want to work live.

 

[FatBob]

Reached a decision, but interested in further comments.

Going to effectively use the 63A MCB's separately

 

[nikmet]

Don't want to touch the DNO fuse, and don't want to work live.

Call supplier and arrange for an isolation switch to be installed before you carry out the work?

 

[Des 56]

Some possibles

Each mcb would need to be linked physically at the very least so they both drop out
You will have a combined rating equivelent to a 126 amp mcb
Each tail would be too small for the combined mcb rating
The mcbs would have a higher rating than the enclosure
It wouldn't be a double pole isolator
Probably the mcbs wouldn't survive a prospective short circuit current


Its probably best staying as just a thought

Its so much easier an option, just to dump the mcbs and fit an isolator, then switch the neutral as welll,no dno no hastle no problem

 

[Octopus]

Are you sure the DNO seal on the main fuse is there?

 

[RogertheBodger]

Are you sure the DNO seal on the main fuse is there?

I can see from here that it has fallen off.

 

[malcolmsanford]

An MCB and an isolator are two physically different things. Most MCB's aren't rated as isolators.

A BS 60898 here in the UK is rated for for isolation, and emergency and functional switches mate. Are they not recognised as such there in SA ?

 

[johnboy6083]

i would like to add that only a dp mcb can give proper isolation in a 1ph installation

 

[malcolmsanford]

i would like to add that only a dp mcb can give proper isolation in a 1ph installation

True mate if you wish to use it as in reg 537.1.4, for both a TN and TT system.

In a TN system an isolator only need to switch the LINE conductor if the neutral is deemd to be conected to a reliable earth at a suitably low impedance.

 

[FatBob]

True mate if you wish to use it as in reg 537.1.4, for both a TN and TT system.

In a TN system an isolator only need to switch the LINE conductor if the neutral is deemd to be conected to a reliable earth at a suitably low impedance.

537.1.4 A main switch .. shall interrupt both live conductors of a single-phase supply