Local motor isolator and early-break contact

A local isolator is a switch next to the motor that is used to isolate the motor from the mains during maintenance work. The switch can be locked in position 0 with a padlock, so that nobody can switch the motor on during maintenance.

Place in the power circuit

The local isolator is the last device in the power circuit before the motor: the supply goes through the motor protection circuit breaker or switch-fuse and the contactor to the isolator, and from there to the motor.

Early-break contact

An auxiliary contact that opens before the main contacts can be fitted to the isolator. When it is wired in series with the contactor control, this is what happens:

When the switch is turned, the early-break contact opens first and the contactor drops out.
The contactor breaks the motor current, so the main contacts of the isolator open without load.
The self-holding is released, so the motor does not start by itself when the isolator is turned back on.

In your region

Showing notes for

UK & Ireland

In the UK this switch is called an isolator. Where mechanical maintenance may involve a risk of injury, BS 7671 requires a way to switch off that stops the equipment from being switched on again by accident, and the device must be clearly identified by its position or by marking. In Great Britain, PUWER also requires a clearly identifiable and readily accessible way to isolate work equipment where appropriate.

Source: IET Wiring Matters: isolation and switching for mechanical maintenance (2023) · UK PUWER 1998, regulation 19: isolation from sources of energy

Europe, Other IEC countries

IEC 60204-1 covers this in clause 5.4, devices for removing power to prevent unexpected start-up, and clause 5.5, devices for isolating electrical equipment.

Source: IEC 60204-1:2016+AMD1:2021 preview (contents and foreword)

Australia & New Zealand

In Australia and New Zealand this is called an isolating switch. AS/NZS 3000 covers shutting down for mechanical maintenance in clause 2.3.6, and AS/NZS 4024.1603 covers the prevention of unexpected start-up on machines.

Source: Kraus & Naimer Australia and New Zealand: isolators explained · Standards Australia: AS/NZS 3000:2018 Wiring rules · Standards Australia: AS/NZS 4024.1603:2019 (ISO 14118:2017 IDT)

United States

A switch like this is called a disconnect or safety switch. The NEC wants a disconnect within sight of the motor; one manufacturer sums this up as within sight and no more than 15 m (50 ft) away.

For lockout, OSHA says pushbuttons, selector switches and other control circuit devices are not energy isolating devices. Lock out a disconnect or circuit breaker instead.

Heavy duty safety switches can have auxiliary contacts that open before the main contacts, like the early-break contact above.

Source: ABB: manual motor starter guide, North American applications · OSHA 29 CFR 1910.147: control of hazardous energy (lockout/tagout) · Siemens: auxiliary contacts for safety switches, catalogue page

United States, Canada

On ladder diagrams the disconnect is marked DISC. NFPA 79 calls it the disconnecting means.

Disconnect, three-pole

Source: NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations) · Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples

Canada

A switch like this is called a disconnect or safety switch. CEC Rule 28-600 requires a disconnecting means for each motor. One manufacturer sums up the distance rule as within sight of the motor and no more than 9 m away.

Heavy duty safety switches can have auxiliary contacts that open before the main contacts, like the early-break contact above.

Source: Electrical Industry News Week: motor rules in CEC Section 28 (2021) · ABB: manual motor starter guide, North American applications · Siemens: auxiliary contacts for safety switches, catalogue page

Frequently asked questions

What is a local isolator?

A switch next to the motor for isolating it from the mains during maintenance. It can be padlocked in position 0, so nobody can switch the motor on.

Where does a motor isolator go in the circuit?

Last in the power circuit before the motor. The supply goes through the motor protection and the contactor to the isolator, and from there to the motor.

What is an early-break auxiliary contact for?

It opens before the main contacts. In series with the contactor control, it drops out the contactor first, so the isolator opens without load. It also releases the self-holding, so the motor does not start by itself when the isolator is turned back on.

Local isolator