Selecting right MCB

[mehdi1351]

Hello,
I am new to the electrical world, but I think choosing a 10A rated MCB ( Schneider model: A9F55310) for a motor with 0.16A full load current would be not a good idea?
The story is, I have a motor (0.1KW, PFC=0.85, 3ph 415V) and I guess the Full Load Current ,FLC=(0.1/0.85)/(1.73*0.415) =0.16A and I noticed a MCB with 10A rated current is installed to protect the overcurrent/overload!!
That was a bit wired to me, hence wanted to double check with you guys?

Thanks

 

[Jraef]

I can't speak to your local Codes in NZ, but this is how it might work here.

It is possible that a motor that small is "self protected", meaning it has it's own thermal protection built-in. If it has a Reset button on it, that's a big indicator, but sometimes it just says "Thermally Protected", or even just "TP" on the nameplate. If so, then the MCB would only need to be there to protect the wiring, so as long as the wire is rated for 10A, you would be good to go.

 

[mehdi1351]

Thanks for the reply. The motor is a small vibrator motor (MVE 100/3E) and nothing on the nameplate!, but in the manual it is mentioned that "
All electric vibrators MUST be connected to a suitable external overload protection. Always use a thermal-magnetic type motor protection, with delayed cut-off, to avoid stopping the motor during start-up when the current draw is higher than the rated running current for a few seconds".
I guess based on the above, the MCB should be set 10-20% of the FLC of the motor with a few seconds delay to avoid unwanted trip during motor start -up. I am right? If this is the case then definitely 10A rated MCB is not protecting the motor, it is only protecting the wires.

 

[Jraef]

Correct, that MCB is not suitable by itself. I suggest using a Motor Protection Switch for this with a dial adjustable overload protection, You would set it at the motor nameplate Full Load Amps, it already has the other issues factored in. Something like this (since you already must have access to Schneider products):

Link

 

[mehdi1351]

Thanks and much appreciated.
Have a great day

 

[TwoBlocked]

I don't think you will find any motor controller with a setting low enough. You may have to go with slow blow fuses.

 

[mehdi1351]

Yap, I decided to keep the 16A MCB, but put a Bimetal overload protection. So the 16A MCB will take care of the wires (in case of over-current) and bimetal will take care of the overload

 

[Jpflex]

I can't speak to your local Codes in NZ, but this is how it might work here.

It is possible that a motor that small is "self protected", meaning it has it's own thermal protection built-in. If it has a Reset button on it, that's a big indicator, but sometimes it just says "Thermally Protected", or even just "TP" on the nameplate. If so, then the MCB would only need to be there to protect the wiring, so as long as the wire is rated for 10A, you would be good to go.

I noticed that when the NEC talks about thermally protected motors it allows the overload to be protected beyond the normal 1.15 to 1.25% of the nameplate FLC. However this makes it appear that an overload device is to be calculated and selected but these often integral devices are pre installed and you would not check its rating from factory?

 

[Jraef]

I don't think you will find any motor controller with a setting low enough. You may have to go with slow blow fuses.

The one I linked to is adjustable from 0.63 to 1.6A.

 

[Jraef]

I noticed that when the NEC talks about thermally protected motors it allows the overload to be protected beyond the normal 1.15 to 1.25% of the nameplate FLC. However this makes it appear that an overload device is to be calculated and selected but these often integral devices are pre installed and you would not check its rating from factory?

The integral motor thermal protection is allowed to be different because it is directly reading the motor winding temperature, as opposed to inferring it based on the current.

 

[Jpflex]

The integral motor thermal protection is allowed to be different because it is directly reading the motor winding temperature, as opposed to inferring it based on the current.

Yes but my point is that NEC tells you that when making selection of overload protective devices the rating selection can be 170% I don’t have book to remember exact. However you are not selecting these because they may be integral and preselected by the manufacture

 

[Jraef]

Yes but my point is that NEC tells you that when making selection of overload protective devices the rating selection can be 170% I don’t have book to remember exact. However you are not selecting these because they may be integral and preselected by the manufacture

Right. And you have no way of knowing, one way or the other, how the manufacturer selected that integral protective device. So you just have to trust that they did their own homework and designed it to meet requirements, which by the way would be the same for UL (assuming the motor is UL listed or recognized), which is something we (electricians) don't typically decide on.

If an AHJ were to challenge that, I would be very surprised... If they see that the motor says it is Thermally Protected, that's all they care about.