MCCB incomers on paneboards

[JameZZZ]

Hi I have a question which I am sure can be answered by the chaps on here in seconds. Why do panelboards often have MCCB's as incoming devices over isolating switches? I understand this can be useful if the PFC is high at the board and an incomer like this can protect outgoing devices. Is this the only reason though? Is there any other reason? Doesn't the addition of an Mccb just cause discrimination problems with the device protecting the supply to the panel board? Am I missing something? Thanks for your replies.

 

[Knobhead]

You don’t say the source of the supply.

 

[JameZZZ]

Hi Tony,

i am am talking hypothetically not about an actual installation. Let's say it is fed on a sub main from another MCCB board (not tails at origin and circuit length is over 3m). What is the purpose of the MCCB incomer? I have seen this many times and always wondered. Is it purely due to fault current restrictions on the panel-board/ outgoing ways?

thanks

 

[guest119]

One example is where the sub main is a fair old length and therefor oversized and wont fit a 125a db isolator, ie anything over 50mm2.

 

[Knobhead]

One thing, I hate is the use of “panel board” it’s an Americanism. Distribution board or just board if you don’t mind.

You mentioned discrimination, our companies policy was as follows:

For a sub board fit an isolator and rely on the main boards outgoing OCPD to protect both the cable and the board’s busbars.

If the board is fed off an intake busbar chamber use a MCCB for the incoming as it will be the OCPD for that board.

The bottom line is, where is the board fed from and what protects the feeder? To fit an MCCB in series with the circuit OCPD is nothing but a waste of money.

Larger cable sizes can be accommodated by termination adaptors. It’s not a reason to fit a MCCB in series with the OCPD at the origin.

There is the odd occasion that E/L protection is required for a board, then a suitable MCCB incorporating E/F as the incoming is be fitted. Definite indication the fault is related to the board and not the cable. It was the only time we deviated from the above. I could count on the fingers of one hand the boards I’ve fitted with E/F protection.

 

[JameZZZ]

Tony,

Thank you for the information. Very Useful for my understanding. Sorry for my poor terminology, I shall use the term 'MCCB board' in future.

The bottom line is, where is the board fed from and what protects the feeder? To fit an MCCB in series with the circuit OCPD is nothing but a waste of money.

What you describe above is what I have seen on may of the jobs I have worked on. I think this may be because the jobs I work on are mainly consultant designed and therefore are over specified? I think I will suggest to my gaffer that we suggest changing to switch isolators on future jobs for a cost saving (on conformation that OC protection is not required because it is afforded upstream).

Thanks again,

JameZZZ

 

[Engineer54]

Some main switches/isolators look very much like MCCB's but have no Over Current Protection within them, they just use the same frame casing as the the MCCB's of the same manufacturers range.

In some cases, depending on the distance from the main distribution board (MDB) or whether the MDB is in a locked or restricted location, it may well be prudent to provide a MCCB or other means of OCPD at a sub/final distribution board!! If it's of an adjustable MCCB type, then the parameters can be pretty much set to cover the DB needs while ensuring discrimination with the up stream feeder OCPD!!

 

[JameZZZ]

Hi E54,

The are definitely MCCB's (60947-2).

If it's of an adjustable MCCB type, then the parameters can be pretty much set to cover the DB needs while ensuring discrimination with the up stream feeder OCPD!!

They are always adjustable and wound down. This must be the thinking.

Thanks very much,

JameZZZ

 

[Knobhead]

Not the way we’d do it but every competent person had access to every part of the system.

I always forget about restricted access switch rooms. We would regard that as unsafe, access could be required at any time in an emergency.

 

[ImpededLoop]

I've used MCCB incomers where a board has been supplied directly from a rising busbar in a tower block.

If I recall correctly, even Eaton (MEM) call them panel boards, but I agree that it's not the best name for them!