JohdotaA switch-fuse is a three-pole switch with fuses inside it. In a motor feeder, it is the first device after the supply. The handle is used to isolate the feeder by hand, and the fuses protect the cable and the equipment against short circuits.
Motor feeder with a switch-fuse
The current flows through the fuses of the switch-fuse to the contactor, and from there through the thermal overload relay to the motor. The tasks are divided like this:
- The switch-fuse isolates the feeder and protects against short circuits.
- The contactor starts and stops the motor.
- The thermal overload relay protects the motor against overload and stops it through the contactor.
The same can be done with a motor protection circuit breaker, which has short-circuit and overload protection in one device. Both ways are widely used.
Fuses
Fuses do not protect the motor against overload, because they must withstand the starting current. That is why a switch-fuse always needs a thermal overload relay.
There are aM fuses made for motor feeders, but general-purpose gG fuses are also used. The fuse size is taken from the table of the contactor and overload relay manufacturer, in which the devices have been tested together. For example, for a 2.5–4 A thermal overload relay, one manufacturer gives aM 6 A or gG 10 A.
Remember
- A blown fuse does not turn the handle to position 0. If only one fuse blows, the motor is left running on two phases. Fuse monitors are available as accessories for detecting a blown fuse.
- Auxiliary contacts can be added to the switch to tell the control circuit the position of the switch.
In your region
Showing notes for
UK & Ireland
British BS 88 motor fuses can be class gM, with two current ratings. For example, a 32M50 fuse carries 32 A continuously and has the time-current characteristic of a 50 A fuse.
Source: Eaton Bussmann: BS 88 fuse 32M50 (gM) · Schneider Electric: Electrical Installation Guide, fuse classes gG, gM and aM
Australia & New Zealand
The Australian adoption of IEC 60269-1 (AS 60269.1-2005) has been withdrawn. Take the fuse type and size from the device maker's tables.
United States, Canada
The North American equivalent is a fusible safety switch. The fuses are UL classes, not gG or aM: for example class J (in Canada HRCI-J), class RK1 and RK5, and class CC up to 30 A. Time-delay class J and RK fuses hold 500% of their rating for at least 10 s, so they ride through the motor starting current.
Source: Littelfuse: POWR-GARD fuse catalogue, application guide · Siemens: heavy duty safety switches, catalogue extract
Europe, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
What is a switch-fuse?
A three-pole switch with fuses inside. The handle isolates the feeder by hand, and the fuses protect the cable and equipment against short circuits.
Does a switch-fuse protect a motor against overload?
No. The fuses must withstand the starting current of the motor, so they only protect against short circuits. Overload protection needs a thermal overload relay below the contactor.
What size fuses does a motor feeder need?
Take it from the contactor and overload relay manufacturer's table, where the devices were tested together. For a 2.5–4 A overload relay, one manufacturer gives aM 6 A or gG 10 A.
Switch-fuse or motor protection circuit breaker?
Both are common. With a switch-fuse, the fuses handle short circuits and a separate overload relay handles overload. A motor protection circuit breaker does both in one device.
What is the auxiliary contact of a switch-fuse for?
It tells the control circuit whether the switch is on or off, for example to block starting or switch on an indicator light.
Can a fuse be changed while the switch is on?
No. The fuse carriers cannot be opened in position I. Change fuses with the switch in position 0.
What does a blown fuse in a switch-fuse do to a motor?
The handle does not move to 0. If only one fuse blows, the motor keeps running on two phases. Fuse monitors are available as accessories.
Sources
- ABB: OS switch-fuses 20–800 A, brochure
- ABB: motor starters with fuses and type 2 coordination
- Schneider TeSys LRD08 2.5–4 A: recommended fuses aM 6 A and gG 10 A (shop page in Finnish)
- Eaton Bussmann: BS 88 fuse 32M50 (gM)
- Schneider Electric: Electrical Installation Guide, fuse classes gG, gM and aM
- Standards Australia: AS 60269.1-2005 (withdrawn)
- Littelfuse: POWR-GARD fuse catalogue, application guide
- Siemens: heavy duty safety switches, catalogue extract