Coordination is possible with series rated devices. It is an old wives tale that the upstream device must operate in order to protect a down stream device, in a NRTL tested series connection.
This is from Square D’s publication, Bulletin No. 0600DB0108 Applying Short Circuit Current and Series Connected Ratings
“As a result of the potential simultaneous opening, some designers of electrical distribution systems believe that all circuit breaker coordination is lost when using series-connected ratings. In reality, using series-connected ratings provides virtually the same system coordination as a fully-rated system…”
The example given is for a 480V system with 30,000A available, the upstream device is an 800A breaker and the down stream device is a 125 device.
“…In both the series-rated and fully-rated systems, coordination is achieved to the same degree. In fact, when a UL Recognized series-rated combination is used, the branch circuit breaker is more likely to be a smaller frame size than in the case of a fully-rated system. The smaller frame size branch circuit breaker will have a different characteristic trip curve and may result in better overall coordination…”
The total available fault current must be taken into account when conducting any coordination study. For high fault levels and similar sized OCPDs coordination may not be possible at all. This is especially true for all circuit breakers, but is even a concern with fuses.
From the Bussmann publication, SPD Electrical Protection Handbook. “Selectivity between two fuses operating under short-circuit conditions exists [only] when the total clearing energy of the load side fuse is less than the melting energy of the line side fuse.”
This is similar to the Square D statement of “In both the fully-rated and series-rated combinations, coordination exists for all values of current below the magnetic instantaneous pickup point of the main (MAL36800) circuit breaker.”
