| Article #kba145pmr |
| Short Circuit Current Ratings of Three-Phase Monitor Relays |
| Issue |
| What is the Short Circuit Current Rating (SCCR) of Macromatic Three-Phase Monitor Relays? |
| Solution/Resolution |
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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. 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 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 circuit. 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. The relay output contacts of Macromatic phase monitors are intended for use within panel control circuits, not within power circuits. 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. The contactor's contacts then operate the load which is part of the power circuit. Individual control circuit components do not need to have short circuit current ratings (SCCR) because they are not used in calculating the rating of the overall panel nor are they part of the power circuit. Refer to the UL508A standard sections SB 3.2.1 and SB 4.4.4 for additional information. You can also click here to review a decision flowchart on whether a component should have SCCR ratings provided by Schneider Electric. |
| This article applies to the following Macromatic products: |
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Last Reviewed: 06/30/2010 |