MCB as MAIN TRIP

[Motonite]

Hi All,

I would like to get some advice on this attached plan.

For reference the 3 phase is for wood working machines will be using 3hp or lower hp wood working machines at 15amp.

I have used a 32amp MCB between the isolator and busbar. Because the here the domestic 3 phase is provided at 30amps and the main wires are rated at 36amps(6mm) for 3 phase. Also the busbar splits into 3 circuits (2) Single phase (1) 3 Phase. Any advice on this?

Thank you.

 

[Andy™]

so... whats the question? why do you have both a single phase & 3 phase supply going into the same DB?

 

[apprentice87]

No earthing arrangements either

 

[Fleeting]

FB1 can be triple-pole and neutral using rcbos for single phase circuits and rcds at the sockets for three phase circuits assuming upstream rcds are not required for additional or fault protection.
Having the 32A mcb at source you will not achieve selectivity between protective devices.

 

[poni]

You don't need the single phase supply , you could remove the cable and RCD ,then feed the isolator from the 3 phase RCD , unless you want to keep two supplies going into the garage,

what is a "16A plug base isolator" ?

why two 3 phase supplies and one single phase supply for the plug base?,
your discrimination looks ok, a 32A feed, then 16A, 20A,MCBS, ( but you will trip this 32A if you fully load any two of the circuits).
remember the RCDS must be 30ma,
I would put the RCD indoors ,so the all the out going cables had RCD protection,

don't forget to tell us about the earthing arrangements either, this could change things,

 

[Fleeting]

You don't need the single phase supply , you could remove the cable and RCD ,then feed the isolator from the 3 phase RCD , unless you want to keep two supplies going into the garage,

what is a "16A plug base isolator" ?

why two 3 phase supplies and one single phase supply for the plug base?,
your discrimination looks ok, a 32A feed, then 16A, 20A,MCBS, ( but you will trip this 32A if you fully load any two of the circuits).
remember the RCDS must be 30ma,
I would put the RCD indoors ,so the all the out going cables had RCD protection,

don't forget to tell us about the earthing arrangements either, this could change things,

Selectivity is not okay there is more to it than overcurrent, what will disconnect first under ADS.

 

[poni]

what will disconnect first under ADS

the RCD, on earth fault, and the MCB on short circuit depending on type

I also said "don't forget to tell us about the earthing arrangements either, this could change things"
it may need to TT

 

[Fleeting]

the RCD, on earth fault, and the MCB on short circuit depending on type

I also said "don't forget to tell us about the earthing arrangements either, this could change things"
it may need to TT

So you are assuming/guessing. This has nothing to do with the earthing arrangement it is about selectivity between protective devices. What will trip first the upstream 32A device or the downstream 20A device under fault conditions.

 

[poni]

What will trip first the upstream 32A device or the downstream 20A device under fault conditions.

that will depend on so much
type of MCB
Make of MCB
the size of the overload
the amount of time the overload takes
total load already before fault on the up stream MCB

But then this is the same with all electrical distribution

I may be Assuming but that is all I have to go on and would ask for more info,

 

[Motonite]

that will depend on so much
type of MCB
Make of MCB
the size of the overload
the amount of time the overload takes
total load already before fault on the up stream MCB

But then this is the same with all electrical distribution

I may be Assuming but that is all I have to go on and would ask for more info,

Thank you all for the info. Will provide some additional details.

Why did I add a MCB at the main?

Reason to add a 32amp MCB is because the domestic supply in Sri Lanka is 30amp and the wires used from the meter till the MCB is rated for 36amp(6mm) for 3 phase. So In case If I had two 2 hp single phase motors running + a 15a appliance which might add up to 30~40a.

Is this a safe idea to the MCB after the main switch?
All the RCD's at rated for 40a. Will this be a issue

Spec of the main MCB:
Schneider A9N4P32C xC60 - 4 poles - 32 A - C curve
View Spec

 

[poni]

Sri Lanka

sorry this is a UK based site and they will have different regs in Sir Lanka and I have no knowledge of them , so sorry I cant help you any more