Can an MCCB Be Used as an Isolator?

What Suitable for Isolation Really Means

Isolation duty, the tests behind the isolation marking, and how the isolation version of the CJMM1 moulded case circuit breaker from C&J Electrical is specified

Open a distribution panel and you will often find one breaker that is expected to do two jobs: protect the circuit, and give the maintenance crew a point where the supply can be cut off and locked. The second job is the isolation function. It looks like a small addition on the nameplate, but it changes what the device has to prove.

The difference is not cosmetic. On a moulded case circuit breaker it decides whether the device can serve as the point of isolation for a circuit, or whether a separate isolating device has to be installed alongside it.

What suitable for isolation means

The isolation function is the ability of a device to create a defined open point in a circuit, so that the section downstream can be worked on without the supply being restored by accident. On a moulded case circuit breaker it is a declared property, not an assumption. A breaker that has not been assessed for it can still open and close, and can still be operated by hand, but it has not been shown to hold a contact gap that the standard accepts as an isolating gap.

The isolation marking is a statement about the device, so it is worth checking on the nameplate and in the catalogue rather than assuming it from the size or the look of the breaker. Isolation duty is also not the same as switching duty. A device can switch a circuit on and off under load without ever having been assessed for isolation.

What a breaker has to prove before it can carry the isolation marking

IEC 60947-3 is the standard that covers switches, disconnectors, switch disconnectors and fuse combination units. It sets out what a device has to demonstrate before it can be marked as suitable for isolation, and the checks look at the open position rather than the closed one.

• The contact gap. An impulse withstand voltage is applied across the open contacts, to show that the gap holds off voltage rather than merely being open.
• Leakage through the open device. The current that can still pass through the open device is measured against limits, so that the downstream circuit is genuinely isolated and not only nominally so.
• The mechanism. The operating mechanism is loaded to show that a sticking or welded contact cannot be presented as an open position on the indicator, and that the device can be locked in the open state.
• The switching duty. The utilization category states the load the device has been type tested to make and break, AC-21 covers resistive loads with moderate overload, AC-22 mixed resistive and inductive loads, and AC-23 motor and highly inductive loads.

The CJMM1 isolation version declares compliance with IEC/EN 60947-3 and lists the isolation function on every frame.

Isolator, switch or switch disconnector?

The three words are often used interchangeably on site, but the standard treats them as different devices. A disconnector, also called an isolator, is an off-load device: it creates the isolating gap, and it is not intended to be opened or closed against load. A switch is the other way round: it makes and breaks current up to its rated category, but it does not by itself carry the isolation marking. A switch disconnector does both, which is why it is the usual choice for a main switch.

The distinction matters when a circuit breaker is considered for isolation duty. A moulded case circuit breaker is first of all a protective device, and its ability to break fault current comes from the overcurrent release rather than from a switching category. Where the same device is also declared suitable for isolation, it can serve as the isolating point for the circuit. Where it is not, a separate isolating device is needed, whatever the handle suggests.

What the isolation function does not replace

An isolation function is a property of the device, not a complete isolation procedure. Whether a circuit can be worked on safely depends on the installation as much as on the breaker: the point of isolation has to be identified, the device locked in the open position, the supply proved dead before work starts, and any stored energy dealt with. A breaker that carries the isolation marking makes that procedure possible. It does not carry it out.

Isolation and protection also stay separate. Isolation protects the person doing the work, while overcurrent protection protects the circuit. A moulded case circuit breaker with an isolation function can cover both duties on the same frame, but they remain separate functions with separate requirements behind them.

Introducing the CJMM1 isolation version

The CJMM1 isolation version is a moulded case circuit breaker manufactured by Zhejiang C&J Electrical Holding Co., Ltd. It is declared to IEC/EN 60947-3 and comes in 3-pole and 4-pole versions across four frame sizes. The isolation function is listed for every frame.

Both a distribution release and a motor protection release are available, and the series offers a choice of wiring and operation. The full ratings are in the table below.

Key technical data of the CJMM1 isolation version:

Note: Confirm the final selection, the point of isolation and the lock-off procedure against the panel design and the applicable installation requirements.

Application scenarios

Main switch of a distribution board. The whole board can be isolated from the supply at a declared isolating point.

Incomer and feeder positions in commercial and light industrial panels.

Motor feeders. The isolation point sits next to the machine, with protection sized for the motor rather than for the cable alone.

Circuits that switch the neutral. A 4-pole breaker with the appropriate N pole type opens the neutral with the phases, or keeps it connected, according to the design.

New-build and replacement work where the isolating point has to be a declared function rather than an assumption.

About C&J Electrical

Zhejiang C&J Electrical Holding Co., Ltd. manufactures low-voltage electrical products, mainly circuit breakers and related protection devices. The company supports OEM brand customization and can ship products as SKD kits for local assembly.

For more information on the CJMM1 isolation version or to request a quotation, please contact us.