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5 built-in variable-speed drive safety features

Variable-speed drives used in motor-driven applications must carry certain safety features in order to protect users and equipment whilst complying with BS EN 60204-1 – the British standard for Electrical Machinery Safety. Here are five typical safety functions for variable-speed drives:

Safe torque off (STO)

This comes built in as standard with ABB drives and is used to bring a drive to a no-torque state - preventing unexpected start-up. In the case of an emergency stop, STO cuts off the drive output to the electric motor which allows it to safely slow to a standstill.

Typical applications: Conveyors and grinding mills

Safe stop 1 (SS1)

SS1 brings an electric motor to a smooth, controlled stop before automatically switching to STO. SS1 suits applications where sudden stopping will cause damage to equipment and products.

Typical applications: Saws, centrifuges and winders.

Safe stop emergency (SSE)

This provides an emergency stop function and can be programmed to activate either STO or SS1 when required.

Typical applications: As above.

Safely-limited speed (SLS)

The SLS feature limits motors to a certain speed. It does this by monitoring the run speed and activating either STO or SSE if it should exceed the defined limit. This is used in applications where maintenance or cleaning staff require access during operation and accidental acceleration presents a safety risk.

Typical applications: Mixers and conveyors.

Safe brake control (SBC)

SBC is used to control holding brakes when a drive is de-activated with STO. This function is typically used in applications where a load is hanging and in a torque-free state.

Typical applications: Cranes, winches and elevators.

Here at Gibbons we’re aware that safety is paramount in industry and as an Authorised Value Provider we’re able to recommend suitable drives and software that help users manage risks on site.

For more information on ABB drives and software, call 01621 868138 or email

Variable-Speed Drives, Electric Motors

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