motor protection

help please, it's not actually plc related forgive me. just need some info for the proper fuse type and rating for protecting a small pump rated at 1.5A 240V. need to be protected for overload and shorts. cheers

I always use dual element time delay "type" fuses for motors. I would interject that "fuses are fuses" and "overloads are overloads". In MOST cases, the appropriate sized fuse for a motor, won't necessarily protect it from an "overload". Conversely, a fuse size that WILL successfully protect a motor from "overloads", will have frequent nuisance trip events.

Stationmaster

Consult the NEC Article 430 for proper sizing of fusing or circuit breakers for an individual motor. (depending on whether it is single phase or three phase consult the proper table for the HP rating of the motor you have)

If you are only trying to protect the motor from overload you can use the nameplate rating and choose the overload according to Article 430. (you can go like 125% if the motor has a service factor of 1.15 or greater and a temp rise of 40 degrees C or less, this is from memory so you should check the code book.)

The FLA current rating of the motor should be taken from the tables to size everything BUT the overloads. The FLA rating for the overloads is the only thing that comes from the nameplate on the motor.

What country is this for?
Is there a limitation on the type of fuse that can be used?

Jim Dungar said:

What country is this for?
Is there a limitation on the type of fuse that can be used?

Click to expand...

it's in the UK. the only limitation maybe would be the size, the fuse holder provided is only 5X20 mm. the main concern is to avoid nuisance tripping.

I do not know of any 5x20mm fuse that will have a sufficient time delay to allow a motor to start and still provide running OL protection. My guess is that you would need to size your fuse to at least 250% FLA for inrush. But on by checking the fuse time-current curve will you know for sure.

In the US National Electrical Code, for appliances and cord-connected devices, there are exceptions that allow the motor short-circuit protection to be handled by the panelboard fuse or breaker, so that you can be legal in some cases with only a motor overload device. Article 430.32: The motor overload device may be: (1) Separate Overload device, (2) Thermal Protector built into the motor, (3)Integral with Motor. A protective device integral with motor if the motor is part of an approved assembly that does not normally subject the motor to overloads.

If the motor is 1 horsepower or less, there are additional exceptions.

For intermittent duty (such as household appliances), the branch-circuit device can be used for motor overload protection also (Article 430.33).

If you are depending on the user's residential branch-circuit fuse or breaker for short-circuit protection, then you need to specify and size your additional fuse for motor overloads (long-time delay), or use a motor with a built-in thermal-trip device.

Of course the UK codes are different.