Using a MCB as a main switch

[mickfred]

My boss has just took over a new site and I went to look the layout. Some boards are damaged some cables need replacing and am sure some other stuff. The site has a old busbar with isolators feed various boards and of her isolators. It also has a board connected directly on to the busbar . The main switch for this board is rated 125a the tails are 35mm ,but the fuse in the maintenance isolators are 250amp DD fuses. So the board and the tails have no protection. My question is (and I know it's late so be gentle) could I swap the main switch for a MCB , or will I have to relocate and feed it off a isolator with correct fuses (the board is a Hager tp and n Invicta3 ) . Now to go home and bed

 

[Andy78]

You say the DB is a Hager invicta 3. If it a 250A board you can get 125A and 250A MCCB incomers. If it's a 125A board they don't do MCCB incomers for them.
An alternative could be a switch fuse on top of the busbar next to the DB.

 

[mickfred]

Yes the board has a 125 incoming kit I was thinking of use a 80a MCB if it will fit or if not move stuff around so I can use the isolator next to it loop a few feet of 35mm over into the top of it to feed it ,remove the current tail from the board and change the fuses to match .

 

[Andy78]

Yes the board has a 125 incoming kit I was thinking of use a 80a MCB if it will fit or if not move stuff around so I can use the isolator next to it loop a few feet of 35mm over into the top of it to feed it ,remove the current tail from the board and change the fuses to match .

You can get 125A incomers for both the 125A and 250A boards, so it could be either. If it is a 250A board you will be able to get a MCCB incomer.

 

[suffolkspark]

Ive had a similar issue before on an EICR..

200A fuses > into busbar chamber > 35mm tails mechanically protected for 2m into DB > db only rated at 125A, so not fusing down for the tails is fine as somebody else said i cant remember reg no., but theres nothing stopping you pulling in excess of 125A through the main switch....

My only options to remedy were to fit a 63A type C mcb.... Or leave it, the current install in question could never overload it so it was left as a C3.

note it had been inspected by quantec who advertise on here 5 yearly twice before i did it and they hadnt even picked it up.

I did buy the mcb though, still sits in my cupboard, its Havells so has probably gone up in value

 

[Leesparkykent]

Am I missing something here? If the 35mm singles are mechanically protected and under 3 meters then there is no requirement for an OCPD to be placed before the DB. Its not really no different to a 100A main fuse in a house with 16mm tails feeding a CU...As long as the load has been assessed, taking diversity into account that the CCC of the 16mm tails wont be exceeded. Or as in the OP the rating of the main switch in the DB. Or have I read the OP totally wrong?

 

[Darkwood]

Unless by design the Dist' Board cannot demand more than the rating of the main switch then as others have suggested it requires over current protection, trying to retro-fit a new incomer is one option as expressed but can be a big job if the board is difficult to raise up to allow the extention box in, if you have room simply fit a switch/fuse at any convenient place about the Busbar chamber and send the tails via it.

 

[Darkwood]

Am I missing something here. If the 35mm singles are mechanically protected and under 3 meters then there is no requirement for an OCPD to be placed before the DB. Its not really no different to a 100A main fuse in a house with 16mm tails feeding a CU...As long as the load has been assessed, taking diversity into account that the CCC of the 16mm tails wont be exceeded. Or have I read the OP totally wrong?

Been industrial you cannot really apply domestic thinking to this situe' if there exists spare ways in the board then you have your first issue, OCPD's need to be fitted unless you can show the load cannot exceed the rating of the tails or main switch, your 3m quote applies to loads that are classed as fixed and/or de-rated at the load end, when you have a fuse board with multiple de-rated fuse ways then this does not omit the risk of accumulative overload, if there are any variable load circuits too this makes it harder to ensure load will remain < cable/mainswitch rating.
I would never tail off a busbar with de-rated cables and switchgear to that of the incomer unless some form of OCPD is fitted to protect the tails.
The only time it would be acceptable is if the sum of the loads in total say in a little 6way lighting board cannot possibly total > than the main switch and tails then this is fine but in industrial environment we tend not to fit little boards as they are wasteful of space, it's the norm' to fit a large 3ph dist' board with plenty of spare ways for future expansion, future proofing boards for additional circuits is normal in design stage and thus it is practically impossible to assess total loading.
From experience I often find industrial circuit are often spec'd incorrectly even by professionals and especially machinery and motor supplies so this is also added reason I would personally always fit appropriate OCPD to protect the distribution board.

I'll add lee you are perfectly correct if we are talking a little F/A isolator or the like where we can class them as fixed loads.

 

[Leesparkykent]

Been industrial you cannot really apply domestic thinking to this situe' if there exists spare ways in the board then you have your first issue, OCPD's need to be fitted unless you can show the load cannot exceed the rating of the tails or main switch, your 3m quote applies to loads that are classed as fixed and/or de-rated at the load end, when you have a fuse board with multiple de-rated fuse ways then this does not omit the risk of accumulative overload, if there are any variable load circuits too this makes it harder to ensure load will remain < cable/mainswitch rating.
I would never tail off a busbar with de-rated cables and switchgear to that of the incomer unless some form of OCPD is fitted to protect the tails.
The only time it would be acceptable is if the sum of the loads in total say in a little 6way lighting board cannot possibly total > than the main switch and tails then this is fine but in industrial environment we tend not to fit little boards as they are wasteful of space, it's the norm' to fit a large 3ph dist' board with plenty of spare ways for future expansion, future proofing boards for additional circuits is normal in design stage and thus it is practically impossible to assess total loading.
From experience I often find industrial circuit are often spec'd incorrectly even by professionals and especially machinery and motor supplies so this is also added reason I would personally always fit appropriate OCPD to protect the distribution board.

Yeah I wouldn’t of designed it like the situation in the OP. I have just refreshed myself on reg 433.2.2 As been said the easiest options would be a switch fuse or MCCB incomer if the board permits.

 

[westward10]

Sounds like it has been tagged on at a later stage with no thought. Busbar systems nearly always have a switch-fuse for protection.

 

[Deleted member 26818]

There is a problem if the DB has the potential to take the current through those 35mm tails over what they can handle and lower than the 250A fuses.

Op - there needs to be some sort of OCPD between the bus bar and the DB. You need to ensure those tails can not be overloaded and if I have read the op correctly those tails, in there current state could be overloaded long before the 250A fuses blow.

No different to a spur off of a RFC.

 

[Nigel]

No different to a spur off of a RFC.

What? Really?

I think you need to go back and read through the thread again.

 

[mickfred]

Took the incoming kit apart and the main switch looks like it can be replaced by a MCB . Ordered a 100a c type and the tail after removing tape on lugs are 50mm . £170 for a MCB. Let's hope it fits

 

[mickfred]

I could have moved it but it would have been a knock on effect and took too long and would of been left with a hole in the case ( then patching up or buy a isolator to cover it up) . Pain . Half the circuits off this board can go , so it will only pull about 5 -20 amps at any time . But as had been said over time who knows .

 

[Darkwood]

@mickfred
Your solution is not a really good method, your board will be required to maintain full discrimination (as it is easily achievable) given the existing install, you will need to front end this board with a minimum 160amp mccb/BS88 to ensure a fault will not trip the actual 100amp switch/mcb as oppose to the circuit under fault.
Designing a solution that creates another problem while it may solve the initial problem is not really an ideal solution. We are talking commercial/industrial here and losing a full board due to a circuit fault through a design flaw can impact on production, business turnover etc and can come back and bite you if the company are made aware of this poor design.

-If you can cancel the 100amp mcb.
-As I have mentioned prior either fit the correct kit (160amp in your case) to replace the front end 125 disconnector.
-Alternatively fit a 160amp switch/fuse or stand alone mccb carrier and route the appropriate tails through it before feeding the fuseboard but this method would still require you to adapt the fuse board to take 160amp input IE the 125 switch has to go but if you can purchase a high rated connection kit that eliminates the disconnector without requiring an expansion box then sorted, any bespoke option you use would have to be fully enclosed with appropriate spacer shields between connections.

If this is the route/line of work you wish to take then you really need to dig down and get your head in the reg's, all credit in raising the concern that you believed this was initially wrong but be careful as industrial design has to be done to the calc' not the rule of thumb often found in domestic, errors in commercial and industrial can cost big for companies and ultimately may lay on your shoulders.

 

[westward10]

This is the big problem with industrial busbar set-ups they rarely allow for expansion at the design stage. How many busbars do you see where there is space for additional enclosures, you normally see remote switch-fuses tagged on using swa cable.