One breaker for multiple motor protection?

[ctpt]

Does IEC allow protection of more than one motor by one common breaker?

I have come across a vendor drawing for a 'Davit' where there are two motors (one for hoisting and the other for slewing, both around 1 hp). And both these motors are protected by one common breaker. Is it ok??

IEC reference would really help!

Thanks

 

[Blackcountryman]

Can you tell if both motors will ever work at the same time.I have seen this on larger cranes where there have been two motions that can never work together and they have similar sized motors. You have one breaker, one vsd and a pair of contactors to select the required motion.
However these have been on much larger machines at least 50kW+
I would have thought for such small drives it is hardly worth it.

 

[Blackcountryman]

I forgot to say I will see what I can find in the standards & installation regs

 

[Blackcountryman]

I've made a couple of assumtions, always dangerous but we have to start somewhere.
a) Your in North America
b) You have one breaker , two contactors and two overloads.

The standard I work to is IEC 60204-1 Safety of Machinery - Electrical Equipment of Machines. I'm told that NFPA 79 is based on this standard or at least has the same intensions.

Section 7.3 O/L protection of motors. States 'Overlaod protection shall be provided for each motor rated at more than 0.5kW' But it does not say indvidual protection.

The let through energy of the breaker(I2t) must be less then the current withstand capability of the weakest cable.

The other area to consider is the co-ordination of the short circuit protection, contactor and thermal overload protection should be co-ordinated, IEC 60947-4-1 Co-ordination with Short Circuit Protective Devices refers. We normally design to Type 2 co-ordination, I belive UL508 has a similar. Most major suppliers, such as Telemecanique, will provide certificates to show the combinations they have tested but I have never seen this for one breaker and 2 motors.

Hope this is of help, sory for the disjointed series of messages.

John