Locked Rotor Currents

To ensure that your safety system complies with the latest specifications regarding motor locked rotor currents, Mertech tested their Class CC switching devices at six different time periods. The details of our contactor ratings with time dependent maximum switching levels are shown below. This will enable you to request the correct size of Automatic Transfer Switch to suit your project.

​​Note to Fire Pump manufacturers:
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As a reminder the BS EN 12845 (2015) says:

The supply shall be designed to meet the combined current of all connected electrically driven fire sprinkler pumps; one in a stalled rotor condition and others at full start current.
Where the building is equipped with both sprinkler pumps and wet riser pumps (see BS9990), the supply should be designed to meet the above and subject of the authorities:

• the wet riser pump starting in a stalled rotor condition 15 second later; and,
• any further wet riser pumps each starting with a further 15 second delay.

Fuses should be capable of carrying the stalled rotor current for a period of not less than 75% of the period needed for the motor windings to fail;
Magnetic and overload trips shall not be used;