Issue:

Why do I measure low resistance on my Smart-UPS step down transformer?

Product Line:

Smart-UPS, Smart-UPS Online

Environment:

All transformer models, All Serial Numbers

Cause:

Transformers e.g. the APC SURT002 will indicate a Low Resistance when measured with a standard Multi Meter. Although this seems strange, it is quite normal as the total impedance isn't taken into account with this measurement.

Inductive Reactance occurs when the current flow in a circuit is reduced due to induction. This is a sort of "magnetic Resistance" and it reduces the flow of current in a circuit, appearing as an energy loss just like resistance. Impedance is made up of Inductive Reactance + Resistance.

Resolution:

In an AC circuit the total impedance consists of the resistance of the conductor and the inductive reactance when AC is applied. However the standard Multi Meter will only measure the normal resistance of the conductor. This value is normally quite low and does not take the Inductive Reactance into consideration.

In some of our transformers such as the APTF10KW01 or the APTF20KW01 the resistance value to Earth (Ground) may also appear low. This is due to the fact that the terminals 7 or 8 (Neutral) are tied to earth to create a TN network.

The customer should be made aware that a special meter capable of measuring total impedance should be used to get a correct value. Should an insulation resistance test be necessary, then the link to earth should be removed.

Released for: Schneider Electric Singapore