I’m just trying to make sense of clause 4.13.1.2 in 3000, where it’s talking about the isolation switch being capable of interrupting the locked rotor current.
‘…in the absence of manufacture information, the locked rotor current shall be taken as 8xFLI for AC motors and 4xFLI DC motors…’
How are these values relevant if the circuit is adequately protected. If the rotor becomes locked and 4 or 8xFLI was generated in the circuit, would this not generally be enough to operate the circuit protection? Is the current not limited by the circuit protection?
Or is it written with the presumption the circuit protection may not operate? Perhaps if there are multiple motors on the one circuit?
Motor Isolation Switch
Re: Motor Isolation Switch
That paragraph applies for a single motor being isolated by the switch.
The following para provides the calculation for switch rating if there are multiple motors isolated by the same switch.
Yes under locked-rotor conditions you would expect the motor's overload protection (as against the circuit overcurrent protection) to operate. Eventually.
Timing depending on the specific nature of the protection(s) compared to the LRC.
Overload protection - for either circuit wiring or motors - generally being thermal (or thermal-based).
Even 8 x FLC doesn't get you anywhere near the (magnetic) short circuit protection part of a circuit breaker's operating curve
Point being that it may be desirable to isolate the motor before the overload protection has operated,
so the switch needs to be capable of safely breaking more than just FLC.
Noting that on many motor circuits the circuit protection device at origin is only there to provide short circuit protection,
with overload protection provided by a different device that
a) operates in all Actives together, and
b) is generally more closely adjustable, and
c) is set much closer to the motor's FLC than the circuit wiring protection can be.
Worked example:
1.5 kW (2 HP) 3-phase motor:
FLC 3 A so LRC taken as 24 A
supplied via 2.5 mm2 ; circuit protected by 20 A mcb
If C-curve, the "short circuit" operation happens between 5.5 x and 8.8 x the mcb's (thermal) rating - so somewhere above 110 A
Even higher for D-curve, above 200 A.
LRC is far below the sorts of current that would provide magnetic operation
24 A is 1.2 x nominal rating of the mcb, so operation will be in approx 1000 seconds.
Good enough to protect the cable, but we don't want the motor to sit there humming that long
- which is why 4.13.2 requires specific overload protection for motors.
But even that takes time to operate.
The following para provides the calculation for switch rating if there are multiple motors isolated by the same switch.
Yes under locked-rotor conditions you would expect the motor's overload protection (as against the circuit overcurrent protection) to operate. Eventually.
Timing depending on the specific nature of the protection(s) compared to the LRC.
Overload protection - for either circuit wiring or motors - generally being thermal (or thermal-based).
Even 8 x FLC doesn't get you anywhere near the (magnetic) short circuit protection part of a circuit breaker's operating curve
Point being that it may be desirable to isolate the motor before the overload protection has operated,
so the switch needs to be capable of safely breaking more than just FLC.
Noting that on many motor circuits the circuit protection device at origin is only there to provide short circuit protection,
with overload protection provided by a different device that
a) operates in all Actives together, and
b) is generally more closely adjustable, and
c) is set much closer to the motor's FLC than the circuit wiring protection can be.
Worked example:
1.5 kW (2 HP) 3-phase motor:
FLC 3 A so LRC taken as 24 A
supplied via 2.5 mm2 ; circuit protected by 20 A mcb
If C-curve, the "short circuit" operation happens between 5.5 x and 8.8 x the mcb's (thermal) rating - so somewhere above 110 A
Even higher for D-curve, above 200 A.
LRC is far below the sorts of current that would provide magnetic operation
24 A is 1.2 x nominal rating of the mcb, so operation will be in approx 1000 seconds.
Good enough to protect the cable, but we don't want the motor to sit there humming that long
- which is why 4.13.2 requires specific overload protection for motors.
But even that takes time to operate.
- Rating: 16.67%