I donâ€™t agree 100% with this statement - "Don't fall into the false perception that a current limitting fuse by itself will limit the available current, 'cause it will not."
For example, Current Limiting Fuse X in the sketch below will reduce the 50 kA available fault current (AFC) at the transformer, to the let-through rating of the fuse, 17 kA, at Switchgear A. The Withstand Rating of the switchgear can then be less than 50 kA, letâ€™s say, 30 kA.
I agree with this statement - "You must have a listed combination to have a series rating."
A series rated system is a listed combination
of circuit breakers, or fuses and circuit breakers, that can be applied at available fault levels above the interrupting rating of the load side circuit breakers, but not above that of the main or line side device (formerly known as a Cascaded System).
The series rated system has only one advantage - lower initial cost compared to a fully rated system.
However, they have many disadvantages that often preclude their use. These disadvantages include:
1. No Selective coordination. The fault shown in the sketch will shut down Panel B.
2. Not permitted for feeders supplying motor loads (if motor load exceeds 1% of the lowest interrupting rating in the series combination).
3. Limited to listed combinations available from manufacturers. Typically applied only for feeder and branch circuit loads.
As a result of these limitations, series rated systems are normally only used for:
1. Lighting feeders and other non-motor applications.
2. Non-critical loads where selective coordination is not needed.
My understanding is that the Y â€“ Z series combination would not be code compliant.
The 10 kA breakers in Panel B are a violation because of the 200 amps of motor loads, but the combination would be OK (when listed)
if the motor loads supplied were less than 1% of the interrupting rating of the downstream breaker Z.