Short Circuit Current Ratings (SCCR) of Three-Phase Monitor Relays

Thursday: April 21, 2016 : Three-Phase Monitor Relays


Issue:

What is the Short Circuit Current Rating (SCCR) of Macromatic Three-Phase Monitor Relays?  

Solution/Resolution: 

When it comes to short circuit current ratings, phase monitor relays are different than most of the control components that go inside of a panel. Standard components like contactors, overloads, fuses, and circuit-breakers all must have these ratings and are often clearly marked accordingly.  This is because they are actually carrying the current, meaning that the current flows through them as part of the systems power circuit. In the event of a short circuit fault, the short circuit current will flow through these devices and therefore, they must be rated appropriately to handle this current.

Macromatic phase monitoring relays monitor the voltage of the three phase power circuit but the current from the power circuit does not flow through the phase monitor. Our phase monitor relays work in the same manner as a voltmeter by measuring the circuit voltage without actually being in the power circuit current path.  In the event of a short circuit fault, the short circuit current does not flow through the phase monitoring relay and therefore no short circuit current rating (SCCR) is required. This exception is defined in the UL 508A standard under SB4.2.1, Exception No. 1. Among other exceptions, it states that voltmeter devices are not required to have short circuit current ratings nor are they considered in calculating the SCCR of a system.

The relay output contacts of Macromatic phase monitors are typically used within control circuits, not within power circuits where short circuit current ratings (SCCR) are required. A typical application would be to use the relay contacts to pickup or dropout the coil of a motor contactor. In this case, the relay contacts of the phase monitor operate the contactor coil which is part of the control circuit. It is the contactor's contacts that operate the load, which is part of the power circuit, and must then be considered in calculating short circuit current ratings (SCCR) for the system. Individual control circuit components do not require short circuit current ratings (SCCR) and are not used in calculating the SCCR of the overall system because they are not part of the power circuit. In the case of plug-in type phase monitor relays, Exception No. 1 of SB4.2.1 includes the relay and socket together as they are evaluated by UL as a relay-socket combination. If your application requires use of the phase monitor relay output contacts within a power circuit, the short circuit current rating (SCCR) of the contacts can be determined using UL508A table SB4.1. Use the table entry for motor controllers 0-50HP yielding a 5kA SCCR. In the case of plug-in type phase monitor relays, the 5kA SCCR includes the relay and socket together as they are evaluated by UL as a relay-socket combination. 

Note: The above also applies to single phase voltage monitoring relays provided by Macromatic.

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