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hi all,

Can anyone provide a worked example of rating a breaker for a circuit.

For example
In an industrial environment, we have our own HV supply and each substation on site has its own earth rod. I assume this falls in to a TT system.

if I wired (14) 8watt lights in parallel back to a DB.

The inrush of the lights together is 2000VA
The length of the cable is 60Meters
1.5mm fp 200
12.10 milli ohms per meter
29mva/m
ambient of 45 degree (derating of 0.79)
Concealed in trunking

I would like to calculate was size breaking capacity device I require....

Helppppppppp
 
hi all,

Can anyone provide a worked example of rating a breaker for a circuit.

For example
In an industrial environment, we have our own HV supply and each substation on site has its own earth rod. I assume this falls in to a TT system.

if I wired (14) 8watt lights in parallel back to a DB.

The inrush of the lights together is 2000VA
The length of the cable is 60Meters
1.5mm fp 200
12.10 milli ohms per meter
29mva/m
ambient of 45 degree (derating of 0.79)
Concealed in trunking

I would like to calculate was size breaking capacity device I require....

Helppppppppp
Assuming it's a TT just because there is an earth rod is a big mistake so the first thing you should do is confirm is it a TT or not?
 
In an industrial environment, we have our own HV supply and each substation on site has its own earth rod. I assume this falls in to a TT system.

if I wired (14) 8watt lights in parallel back to a DB.

I would like to calculate was size breaking capacity device I require....

Helppppppppp

Don’t assume anything, you’ll only make yourself look silly.
Having earth electrodes connected to the substation tells you very little about the earthing system other than the neutral is likely to be connected to earth. The most likely earthing system in use here is TNS.

The breaking capacity of the device will depend on the fault current at the DB it is to be installed in, you may be able to take the breaking capacity of upstream devices into account.
 
Fourteen, 8w emergency lights and a Type B mcb won't do the job?
 
No mention of the type of fitting so 8 watt equivalent LED might have a large inrush current on start up. 14 of them could trip a B type.
 
Private LV distribution systems will be TN-S, IT resistance earthed or earth free. They will not be TT.


LV IT or earth free systems in the in the UK are rare beasts and should be avoided wherever possible. If I remember right permission has to be obtained from the Home Office for such systems. I’ve worked on earth free systems, they are a nightmare.
 
He stated the inrush current was in excess of 2000 amps which is significantly more than 14x8 so no a type B mcb will not do for that scenario

because the inrush current if all the lights are turned in together will trip a type B mcb

He has stated that the inrush current is 8.4A, this is less than 30A (the 5x6A of a 6A type B mcb)
 

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