how to select a correct rated RCCB

[isurunalaka]

hi, i have below applications to be powered up using a 400V 3P power supply. MCCB and RCCB rating have to be decided. pls can you tell me how to select proper RCCB residual current rating for this.

100mA RCCB can be used for this or 300mA RCCB is required for this?

below is the loads to be powered up.

DOL Starter 0.55 kW-Schneider ( France ) - 1
DOL Starter 0.37 kW-Schneider ( France ) - 1
DOL Starter 1.5 kW -Schneider ( France ) - 1
DOL Starter 2.2 kW -Schneider ( France ) - 2
DOL Starter 4 kW-Schneider ( France ) - 1
S/D Starter 7.5 kW -Schneider ( France ) - 2

 

[chuckey]

From my limited knowledge, the first thing to have a look at is the fault current that would flow if you had a L-N short. It could be 40 kA for an industrial site down to 2.5 kA for domestic use.
The running current of the motor will be exceeded while its accelerating a load such as a big fan or conveyor belt. a type A is fast , B slower, you need a C or slower (K is good but rare).
Providing an earth leakage current does not raise the frame of the motor to 50V (300mA X earth resistance), then a 300 mA would do, BUT you could have a continuous earth leakage of 299 mA, with out it tripping but you will have to pay for this waste of power.
So .55kW/1.7 X 400 = 550/692 = .79 A/phase, add on 20% for safety, gives a nice 1A/ph.
Frank

 

[isurunalaka]

thanks for the reply. but why did you reject 100mA? any theory behind that?

 

[chuckey]

The more sensitive the trip of an RCCD, the more likely it is to trip on spurious spikes coming down the mains. I had this at home, I used a 30mA on the incoming mains but it used to trip once or twice a year after I changed it to 100 mA, no more problems.
The 300 mA X earth resistance < 50 V is the safety factor you must work to. You did not say if you are going to one RCCD per motor or one for the whole installation.Also RF filters on power supplies "leak" current from live to earth, so you can end up with a background earth current even when the insulation of all the kit is OK.
Frank

 

[isurunalaka]

is there any thumb rule or something to choose 100mA or 300mA RCCB

 

[chuckey]

See :- **broken link removed** issue 15 "testing of RCDs"
Frank