AC-1 or AC-3? Help

These people can help. 32A isolators are generally available. Telephone?

If in doubt, consult a local Electrical Engineer.

To be more specific the motor is 15KW and 28A and I need an isolator for this. Firstly do I need AC-1 or AC-3? and secondly does the isolator need to be above 28A so 32A for example as been suggested above or do i need to have an isolator 5-7 times higher than 28A (full load current) as the motor is an induction motor and this is required for it to start.

I will assume that this isolator is not the primary means of motor control, and that you are also utilising some form of starter for overload and no-volt protection etc.

The main difference between AC-1 and AC-3 devices is their ability to contain arcing when the switch opens under load. AC-1 switches should only be used where the power factor of the device being switched is 0.95 or higher.

Inductive loads such as motors can produce considerable arcing as switch contacts open, and very high inrush currents on starting. For this reason your primary controller must be at least AC3 rated.

If the isolator in question is simply a service isolator to prevent the motor starting when being serviced, then it will not be opening or closing under load and could then be only AC-1 rated but would be of a higher current rating to handle the larger inrush current. AC-3 would still be the better choice.

For an application without frequent start or reversing, probably AC-3, see -

https://en.wikipedia.org/wiki/Utilization_categories

Example Schneider isolator document for download at following site -

https://www.schneider-electric.co.uk/en/product-range-download/674-tesys-gs/#tabs-top

- specifically "TeSys catalogue - 2019 - 2020 - chapter B5 Switch-disconnector fuses TeSys GS (Version 6.0)"

page B5/16 states that for 32 amp isolator, 32amp rating applies for AC23 duty 400V AC (switching motors).

But it all depends on isolator make & its local standard. To be correct, you should select an isolator which will take fuses of rating/type recommended for motor by its maker and is specified suitable for motor start/stop duty at your motor current by isolator maker.

The following Bussmann document gives 63 amp or 32M63 amp [motor duty fuse of 32 amp physical dimension but 63 amp for short time fusing] for 415V 15kW shaft power motor.

http://www1.cooperbussmann.com/pdf/ad7559aa-0f86-466f-ab3a-da503bf50937.pdf

You would have to check with isolator maker that 32M63 is acceptable - a motor starter must have a thermal overload at <= 32 amp to observe the long term thermal rating of isolator & motor.