Discuss what type mcb in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

W

wattsup

Help please.
I would like to limit a generator output to 32 amps, if over then the main mcb supply will trip if prolonged, and thus protect the generator from overload. On start, a type b mcb will probably trip.
I'm unsure which breaker would be best suited.
This is an auto start genset of 9.5kva, so it would give 35amps or thereabouts at full load. The auto transfer switch has built in electonics time delay etc that enables connection only when the generator is running stable output.

I would like to limit to 30/32amps at full load. The genset is modified with neutral/earth link at supply point/isolator.

I would also like to protect supply from the genset with an rcd, or maybe an rcbo can this be done?
What would be the best option to avoid nuisance trips and what type breaker
Thanks in advance
 
If you want to limit the output of the genny what about putting a 32amp C type mcb on the outgoing side. This will cope with the majority of items connected, however if you are going to put anything on it that has a high inrush use a D type.
 
I dont understand how this will limit the output.

A type B 32A takes 160A to trip instantaneously, at 45A or less, it will NEVER trip (theoretically), and at just over 45a would take 10,000 seconds to trip

you could easily have a sustained overload of 40+ amps, and your breaker would never pop.

why not go the other way - it breaks the deisgn rules, but a 25A type B would flip at about 35A.

Of course the real way to limit the output is to match the loads to it, but that is not always possible, with mobile kit.

I presume the neutral/earth link is to change the genny from a 'floating' power supply to a conventional one, be careful if there is already breakers fitted, they are often matched to account for the 'half' fault voltage

not sure about the neutral/earth lionk being at the isolator either, you are effectively keeping the genny itself as 'floating' and just playing with the output - you would still not have a fault current path back to the 'neutral/earth' of the genny (i know it s not really a neutral/earth but its the best term i could think of!)
 
Last edited by a moderator:
Help please.
I would like to limit a generator output to 32 amps, if over then the main mcb supply will trip if prolonged, and thus protect the generator from overload. On start, a type b mcb will probably trip.
I'm unsure which breaker would be best suited.
This is an auto start genset of 9.5kva, so it would give 35amps or thereabouts at full load. The auto transfer switch has built in electonics time delay etc that enables connection only when the generator is running stable output.

I would like to limit to 30/32amps at full load. The genset is modified with neutral/earth link at supply point/isolator.

I would also like to protect supply from the genset with an rcd, or maybe an rcbo can this be done?
What would be the best option to avoid nuisance trips and what type breaker
Thanks in advance

The MCB is only there to protect the cable/equipment that is connected to the genny and not the genny as Shakey said, so by putting in a Type C (assuming a 32a MCB) would mean 320 amps to flow before it would trip and if a Type D was fitted then over 640 amps would flow before the MCB would trip but I think lots of smoke and a big bang would happen before either of them tripped.:eek::eek::eek:

You will need to make sure what ever you connect does not exceed the maximum output of the genny.
 
Last edited:
The MCB is only there to protect the cable/equipment that is connected to the genny and not the genny as Shakey said, so by putting in a Type C (assuming a 32a MCB) would mean 320 amps to flow before it would trip and if a Type D was fitted then over 640 amps would flow before the MCB would trip but I think lots of smoke and a big bang would happen before either of them tripped.:eek::eek::eek:

You will need to make sure what ever you connect does not exceed the maximum output of the genny.

well its also there to protect the genny as well Ian, its effectively the current limiting device for the circuit.

At the sort of currents we are talking about, the prime mover would normally stall before you could do too much damage, and generally, as soon as the speed drops, under frequency protection will kick in and drop the o/p anyway.

We used to have major problems with gennys, because the Ze was so high, that it would stall before it gave you enough PFC to throw any sort of trip

and remember the currents you are talking about are the instatnaeous trip currents, which awhich, as you know affect the electro-magentic section of the MCB, i am referring to the thermal element currents, which are effectively the same regardless of the type of MCB.
 
Last edited by a moderator:
Thanks Shakey, the genny is for stand-by power only. The kit will not be used for other purposes, ie portable tools etc. So the genny arrives with floating earth, if used for standby the manufacturers reccommend, fitting E&N link at the supply point, in this case a 32amp socket, they say fit the link at the plug not the socket.

They say this will leave the generator unmodified, and warrenty etc will remain valid.
An earth spike is needed for generator framework.

This is of course compatible with existing supply....

My intentions were to do as advised regards the E&N link but also fit an isolator, in effect the plug-in supply would feed the isolator. (this is purely for access problems, I can fit an IP rated isolator, hence easier to disconnect...

I then thought maybe I could kill two birds with one stone, use an mcb, rated at under the max output of the genset (in this case 35 amps or thereabouts at full load)

So I was going to use a 32 amp breaker, but I have been told this will trip on gen start up, if type b. I can't fit the breaker on load side of transfer switch (when voltage is stable etc) because this side is used on normal utility supply also, via contactors etc, so the main supply would then be limited to 32amps.

Would an hrc fuse be better? Maybe a 30amp? Would the initial surge current leave the hrc opperational, but break if prolonged overload, say 2 minutes?
 
Thanks Shakey, the genny is for stand-by power only. The kit will not be used for other purposes, ie portable tools etc. So the genny arrives with floating earth, if used for standby the manufacturers reccommend, fitting E&N link at the supply point, in this case a 32amp socket, they say fit the link at the plug not the socket.

They say this will leave the generator unmodified, and warrenty etc will remain valid.
An earth spike is needed for generator framework.

This is of course compatible with existing supply....

My intentions were to do as advised regards the E&N link but also fit an isolator, in effect the plug-in supply would feed the isolator. (this is purely for access problems, I can fit an IP rated isolator, hence easier to disconnect...

I then thought maybe I could kill two birds with one stone, use an mcb, rated at under the max output of the genset (in this case 35 amps or thereabouts at full load)

So I was going to use a 32 amp breaker, but I have been told this will trip on gen start up, if type b. I can't fit the breaker on load side of transfer switch (when voltage is stable etc) because this side is used on normal utility supply also, via contactors etc, so the main supply would then be limited to 32amps.

Would an hrc fuse be better? Maybe a 30amp? Would the initial surge current leave the hrc opperational, but break if prolonged overload, say 2 minutes?

well with a prolonged overload, like i said the bullshrush charts show the same trip time for a type B, C &D, (about 10000 seconds on 1 and a half times overload), it is the instananeous trip time (and current ) which changes, which is what the start up current will be

Although, if the genny is used on stand by power, i presume the response time will be set to allow it get up to stabilsed Hz and V before allowing it to feed the main system, so why is the surge being caused?

to be honest I DONT LIKE this idea of strapping neutral to earth at the plug ( or socket)

you effectively have a 115V potential fault voltage, which you are holding at zero one side, forcing the other to 230V, i know this happens anyway, but it dont sit right with me - you wouldnt do it at a power station!

obviously any RCD used will have to be after the link (erm...obviously)!
 
Last edited by a moderator:
Shakey would you put the link on the star point alternator?
I don't understand what difference it makes where the link goes, other than leaving the gen unmodified the way I was intending.

RCD's are used on the existing supply dis board, so I have to provide a n&e link somewhere on gen supply, I though it made sense to provide the link at the plug-head, like advised. But now you have me thinking....
 
Shakey would you put the link on the star point alternator?
I don't understand what difference it makes where the link goes, other than leaving the gen unmodified the way I was intending.

RCD's are used on the existing supply dis board, so I have to provide a n&e link somewhere on gen supply, I though it made sense to provide the link at the plug-head, like advised. But now you have me thinking....


thats exactly what i would do, link the star point.

Think about it. Your genny would not have a Ze, as there would be no live to earth fault current path, you would be effectively providing a 'remote' Ze and fault path AFTER the genny output - like i said, it just dont sit right!:rolleyes:
 

Reply to what type mcb in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock