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[Hawk]

Hi guys,

Here is the set up in a shop:

Three phase supply from the supply meter into escos. From the escos there is tails going to a 100A isolator which supplys a board upstairs and a set of tails supplying a 100A 100mA RCD. From the RCD into a distribution board downstairs. From this distribution board there is a cable supplying another board from a 63A Type C MCB.

The problem is that one of the machines in the shop tripped off during a cycle. It first took out a 40A MCB supplying it. I checked out the motor, compressor etc in the machine which were all sound. When turned back on it ran for a few hours and then took out the 63A MCB supplying the board. Im thinking that it is overloaded as there is four freezers, a cooker, four machines which have a total rating of around 62 A aswell as lights sockets off this board. It's funny thou because the shop was a new installation about three years ago and there has not been any previous problem. The owner says that there has not been any new equipment.

The system is a TT system.

I was thinking of getting another 100A isolator to supply the distribution board and doing away with the 63A MCB.

What would you recommend? Also why would the electrician who wired it have used a 100A 100mA RCD instead of A 100A 30mA?

Thanks

 

[nickblake]

it sounds to me like a faulty machine ,motor breaking down underload ,or a faulty MCB ,as you have said its quite heavely loaded so perhaps the MCB's worn ,whats the total load if diversitys taken into account the 100ma was very common on TT systems in 15th edition but an update in the mid 80s made them non compliant

 

[Hawk]

The total load with diversity taken into account is around 40A. But there is times when all the machines run at the same time so to allow for this I was thinking maybe a Type D MCB. I was at the wholesalers today and asked if they had a 100A MCB (MEM) but apparently the biggest is 63 A.

 

[Andyfm]

Can you put the downstairs supply on a separate switch fuse - you can get a cheapish one for about £50 and will take a 80 or 100 amp fuse if your supply cable is big enough

 

[nickblake]

biggest problem with the D type is hitting the maximum ZS , may be an idea to use a clamp meter and check the loadings do you know what the supply fuses are because if they are 60 per phase its possible that you may blow one of them , its the its worked fine before that causes concern ,why has it just started tripping , realy it sounds to me like they need to use the three phase as if your loading up a single phase to 62 amps yu could be causing an inbalance on the other phases