Iso Phase Bus, Switchgear, & Other High Capacitance Loads

Testing Iso Phase Bus (IPB) or the bus work within Switchgear (SG) cabinets, often several in series, is normally an AC Withstand test, where the test voltage is applied from bus to ground and held for 60 seconds. The insulators holding the bus off of ground are either good or defective. If the test voltage holds for 60 seconds with no arcing, the bus duct and its insulators are deemed to be good. DC voltage testing is sometimes performed but is not the technically proper way to test bus insulators and not preferred by their vendors except as a last resort if the user has no AC method available. DC fails to stress the bus similarly to in-service AC voltage conditions, tends to over read surface resistance of dirty or moist insulators, and is influenced by always changing environmental conditions. Also, there are no accepted standards for DC leakage current measurements indicating the health of bus insulators. Generally, DC leakage current readings when testing IPB and SG are not meaningful and can indicate false failures.

IEEE and other standards exist that define the test voltages required and the maintenance manuals from product vendors list the test voltages based on the nameplate voltage rating of the gear.  AC field tests after installation and into the future are usually performed at 75% of the factory test voltage. For standard ratings of IPB and SG, the following test voltages are typical:

Iso Phase Bus Withstand Test Voltages

Test voltages at 75% of Factory for 60 seconds

Switchgear Withstand Test Voltages

Download the PDF for a table from NETA Acceptance Testing Manual