Circuit breaker curves indicate that only a 32A breaker will discrim with 100A fuses. 40A and 50A close but not quite. Is a 32A breaker the max that can be used under 100A fuses in order to properly discriminate and comply with the above clause."
Answer provided by Ian Richardson, Senior Product Engineer at ABB Australia.
To achieve proper discrimination between an upstream fuse and a downstream circuit breaker as required by AS/NZS3000 Clause 2.5.7.2.3 (c) can be difficult for larger sizes of DIN circuit breakers. This clause expressly requires discrimination between the overload curve and the instantaneous setting of the circuit breaker with the fuse curve.
As you have noted this is quite difficult when we are considering a 100A fuse with a MCB size greater than 32A.
For ABB MCB's we publish a coordination table for a 100A upstream fuse with downstream MCB's up to 63A however this is only a partial selectivity. This means discrimination is achieved for MCB sizes (B and C curve) but only for low short circuit values (6kA up to 16A, 5kA for 20-25A, 4kA for 32 to 40A and 3.5kA for 50-63A). The AS/NZS3000 clause does not specify if the protection should be total or partial selectivity so these selections could be valid.
Under partial selectivity if the short circuit value is greater than values indicated above, then the fuse is likely to blow in addition to the MCB tripping, which may interfere with other parts of the installation.
To achieve proper discrimination between an upstream fuse and a downstream circuit breaker as required by AS/NZS3000 Clause 2.5.7.2.3 (c) can be difficult for larger sizes of DIN circuit breakers. This clause expressly requires discrimination between the overload curve and the instantaneous setting of the circuit breaker with the fuse curve.
As you have noted this is quite difficult when we are considering a 100A fuse with a MCB size greater than 32A.
For ABB MCB's we publish a coordination table for a 100A upstream fuse with downstream MCB's up to 63A however this is only a partial selectivity. This means discrimination is achieved for MCB sizes (B and C curve) but only for low short circuit values (6kA up to 16A, 5kA for 20-25A, 4kA for 32 to 40A and 3.5kA for 50-63A). The AS/NZS3000 clause does not specify if the protection should be total or partial selectivity so these selections could be valid.
Under partial selectivity if the short circuit value is greater than values indicated above, then the fuse is likely to blow in addition to the MCB tripping, which may interfere with other parts of the installation.
- See more at: http://www.voltimum.com.au/content/fuse-and-circuit-breaker-coordination#sthash.FKPc17KY.dpufTo achieve proper discrimination between an upstream fuse and a downstream circuit breaker as required by AS/NZS3000 Clause 2.5.7.2.3 (c) can be difficult for larger sizes of DIN circuit breakers. This clause expressly requires discrimination between the overload curve and the instantaneous setting of the circuit breaker with the fuse curve.
As you have noted this is quite difficult when we are considering a 100A fuse with a MCB size greater than 32A.
For ABB MCB's we publish a coordination table for a 100A upstream fuse with downstream MCB's up to 63A however this is only a partial selectivity. This means discrimination is achieved for MCB sizes (B and C curve) but only for low short circuit values (6kA up to 16A, 5kA for 20-25A, 4kA for 32 to 40A and 3.5kA for 50-63A). The AS/NZS3000 clause does not specify if the protection should be total or partial selectivity so these selections could be valid.
Under partial selectivity if the short circuit value is greater than values indicated above, then the fuse is likely to blow in addition to the MCB tripping, which may interfere with other parts of the installation.
- See more at: http://www.voltimum.com.au/content/fuse-and-circuit-breaker-coordination#sthash.FKPc17KY.dpufTo achieve proper discrimination between an upstream fuse and a downstream circuit breaker as required by AS/NZS3000 Clause 2.5.7.2.3 (c) can be difficult for larger sizes of DIN circuit breakers. This clause expressly requires discrimination between the overload curve and the instantaneous setting of the circuit breaker with the fuse curve.
As you have noted this is quite difficult when we are considering a 100A fuse with a MCB size greater than 32A.
For ABB MCB's we publish a coordination table for a 100A upstream fuse with downstream MCB's up to 63A however this is only a partial selectivity. This means discrimination is achieved for MCB sizes (B and C curve) but only for low short circuit values (6kA up to 16A, 5kA for 20-25A, 4kA for 32 to 40A and 3.5kA for 50-63A). The AS/NZS3000 clause does not specify if the protection should be total or partial selectivity so these selections could be valid.
Under partial selectivity if the short circuit value is greater than values indicated above, then the fuse is likely to blow in addition to the MCB tripping, which may interfere with other parts of the installation.
- See more at: http://www.voltimum.com.au/content/fuse-and-circuit-breaker-coordination#sthash.FKPc17KY.dpufTo achieve proper discrimination between an upstream fuse and a downstream circuit breaker as required by AS/NZS3000 Clause 2.5.7.2.3 (c) can be difficult for larger sizes of DIN circuit breakers. This clause expressly requires discrimination between the overload curve and the instantaneous setting of the circuit breaker with the fuse curve.
As you have noted this is quite difficult when we are considering a 100A fuse with a MCB size greater than 32A.
For ABB MCB's we publish a coordination table for a 100A upstream fuse with downstream MCB's up to 63A however this is only a partial selectivity. This means discrimination is achieved for MCB sizes (B and C curve) but only for low short circuit values (6kA up to 16A, 5kA for 20-25A, 4kA for 32 to 40A and 3.5kA for 50-63A). The AS/NZS3000 clause does not specify if the protection should be total or partial selectivity so these selections could be valid.
Under partial selectivity if the short circuit value is greater than values indicated above, then the fuse is likely to blow in addition to the MCB tripping, which may interfere with other parts of the installation.
- See more at: http://www.voltimum.com.au/content/fuse-and-circuit-breaker-coordination#sthash.FKPc17KY.dpufHave an ask the expert question? Try Voltimum's experts now!