No Overload Protection on DB wired to Busbar

[HoverSPRX]

Hi all,

I recently carried out an EICR on a biolabs, it has a DB in the mains room that was wired direct into the busbar on 25mm tails within 3M. The busbar is on a 200A fuse and the DB in question is a 24 way three phase board.

I know that 20 + years ago when this was installed some guy has probably done his calculations and determined that this may not reach the full loading of the circuits originally installed and deemed it ok by diversity. I don't think I would personally replace overload protection with calculations, but this is what is here now.

There has obviously been additions over the years, there are multiple ring mains, heavy loads of fluorescent lighting in constant use, electric water heaters and 32A Commando Socket supplies and misc others. I think the potential is clearly over and beyond what these tails are capable off for a prolonged load.

I'm about to send a quote for a new 100A TPN Eaton switch fuse for it, along with the other remedial works, but just wanted to be sure of my thoughts and solution by running it past some of you. That way when some suit challenges why its costing them so much to get a satisfactory report I will answer with confidence!

 

[westward10]

It should have been given circuit protection from day one because I also doubt the main switch of the board is rated at 200A.

 

[HoverSPRX]

Thought as much, no its only 125. Anything else you think I may have overlooked or straight in with the switch fuse and problem solved?

 

[westward10]

Fit 100A switch fuse and upgrade conductors to 35.0 because 25.0 are undersized for 100A.

 

[HoverSPRX]

Nice one, thanks westward appreciate the quick response.

 

[Darkwood]

25mm tri rated singles, flexible, smaller and quicker to install at relatively similar costs, for 100amp fuse... dependent on loading ypu may need to swap out and adjust main switch and suggested tails size.

 

[davesparks]

Most DBs can have an MCCB fitted as an incoming device, is that definitely not the case in this situation? And could it be an option for the remedial work?

 

[Marvo]

Your question may have its answer outside of the 7671 general regs.

The 60204 regs allow exceptions under certain circumstances with declaring a fault free zone where extra measures have been taken to reduce the risk of fire and physical damage etc.


BS EN 60204-1
7.2.8 Location of overcurrent protective devices
An overcurrent protective device shall be located at the point where a reduction in the crosssectional area of the conductors or another change reduces the current-carrying capacity of the conductors, except where all the following conditions are satisfied:
– the current carrying capacity of the conductors is at least equal to that of the load;
– the part of the conductor between the point of reduction of current-carrying capacity and
the position of the overcurrent protective device is no longer than 3 m;
– the conductor is installed in such a manner as to reduce the possibility of a short-circuit,
for example, protected by an enclosure or duct.

If this was the case then you'd need to establish if the criteria is still met after the alterations and additions that have happened since the installation before you could motivate for a design change.
.

 

[davesparks]

Your question may have its answer outside of the 7671 general regs.

The 60204 regs allow exceptions under certain circumstances with declaring a fault free zone where extra measures have been taken to reduce the risk of fire and physical damage etc.


BS EN 60204-1
7.2.8 Location of overcurrent protective devices
An overcurrent protective device shall be located at the point where a reduction in the crosssectional area of the conductors or another change reduces the current-carrying capacity of the conductors, except where all the following conditions are satisfied:
– the current carrying capacity of the conductors is at least equal to that of the load;
– the part of the conductor between the point of reduction of current-carrying capacity and
the position of the overcurrent protective device is no longer than 3 m;
– the conductor is installed in such a manner as to reduce the possibility of a short-circuit,
for example, protected by an enclosure or duct.

If this was the case then you'd need to establish if the criteria is still met after the alterations and additions that have happened since the installation before you could motivate for a design change.
.

60204-1 applies to the electrical equipment of machines, and only that. This is firmly in the territory of bs7671.

 

[westward10]

25mm tri rated singles, flexible, smaller and quicker to install at relatively similar costs, for 100amp fuse... dependent on loading ypu may need to swap out and adjust main switch and suggested tails size.

I agree they are easier to install but whether the termination is easier is another matter. Size wise I think, not checked you may still have to move to 35.0 if you use the 70° rule for the conductors.

 

[Ian1981]

Regulation 433.2.2 would have allowed the downstream mcbs to act as the overload protection so when it was first installed the tails need not have required overload protection due to the original loading requirements

 

[westward10]

Regulation 433.2.2 would have allowed the downstream mcbs to act as the overload protection so when it was first installed the tails need not have required overload protection due to the original loading requirements

It is a dist board and should have been given circuit protection from day one unless its switch and supply conductors were rated at 200A.

 

[Ian1981]

 

[Toneyz]

Have you done any measurement for the peak current loading via logger, clamp meter etc. IMO the supply tails need upgrading to 35mm2 and the main D.B. switch changed to a mccb to protect these (what is the D.B.) or switched fuse fitted. A consideration may be to change lighting to LED.

 

[The Ghost]

So what am I missing here? If I read it right 200a on a cable rated at 131a in free air or 136 per table above. I can not think of any reason for me to arrange that potential fault on to a cable that is not capable of taking it... would I? you ?

 

[Ian1981]

It is a dist board and should have been given circuit protection from day one unless its switch and supply conductors were rated at 200A.

It is a dist board and should have been given circuit protection from day one unless its switch and supply conductors were rated at 200A.

its the same as when you tap of a large busbar with a smaller CSA of cable overload is located on the load side of not more than 3meters.
If originally the load profile wasn't going to exceed the rating of the main switch then it would have been acceptable.

In this case then yes it now may not be

 

[davesparks]

Regulation 433.2.2 would have allowed the downstream mcbs to act as the overload protection so when it was first installed the tails need not have required overload protection due to the original loading requirements

that regulation allows the device protecting a conductor to be placed downstream of the point in reduction of csa, this can be applied to a switchfuse fed from a busbar as the switchfuse contains a device which protects the conductor.
You can apply this to a DB as it does not contain a device which protects the cable, it contains many devices. You can't just add up the ratings of all of the protective devices in the DB because they will not operate together as a single device to protect the reduced case cable. The DB would need to have a protective device as its income, for example an MCB or MCCB in place of the main switch.

 

[davesparks]

View attachment 36713

That table is unlikely to be of any use as it is for a conductor operating temperature of 90C, the DB incoming terminals will almost certainly be rated for 70C operation and so the conductor operating temperature must be limited to 70C

 

[westward10]

Plus grouping factors are required for tri-rated.

 

[westward10]

that regulation allows the device protecting a conductor to be placed downstream of the point in reduction of csa, this can be applied to a switchfuse fed from a busbar as the switchfuse contains a device which protects the conductor.
You can apply this to a DB as it does not contain a device which protects the cable, it contains many devices. You can't just add up the ratings of all of the protective devices in the DB because they will not operate together as a single device to protect the reduced case cable. The DB would need to have a protective device as its income, for example an MCB or MCCB in place of the main switch.

Mccbs although they would perform the task are rarely used it is nearly always a switch fuse disconnector.