x/EN AP/Na7
-66 MiCOM P74
11. AUXILIARY SUPPLY FUSE RATING
In the Safety section of this manual, the maximum allowable fuse rating of 16A is quoted. To
allow time grading with fuses upstream, a lower fuselink current rating is often preferable.
Use of standard ratings of between 6A and 16A is recommended. Low voltage fuselinks,
rated at 250V minimum and compliant with IEC60269-2 general application type gG are
acceptable, with high rupturing capacity. This gives equivalent characteristics to HRC "red
spot" fuses type NIT/TIA often specified historically.
The table below recommends advisory limits on relays connected per fused spur. This
applies to Px40 series devices with hardware suffix C and higher, as these have inrush
current limitation on switch-on, to conserve the fuse-link.
Maximum Number of Px40 Relays Recommended Per Fuse
Battery Nominal Voltage 6A 10A Fuse 15 or 16A Fuse Fuse Rating > 16A
24 to 54V
2 4 6 Not permitted
60 to 125V
4 8 12 Not permitted
138 to 250V
6 10 16 Not permitted
Alternatively, miniature circuit breakers (MCB) may be used to protect the auxiliary supply
circuits.
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