What is transformer continuity?
Each winding of a series isolation transformer must be an unbroken path. With the transformer isolated and disconnected, a low-resistance meter should read low across each winding and open between them.
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Transformer continuity means each winding of a series isolation transformer is an unbroken conductive path. With the circuit isolated, locked off, proven dead and the transformer disconnected, a low-resistance meter across the primary and across the secondary should read low and steady, while a check between the primary and secondary should read open.
The primary matters most because it carries the whole loop current: an open primary breaks the series circuit and puts every fitting out. An open secondary leaves only its own fitting dark, but can develop a dangerously high voltage across its open leads and overheat the transformer. Good continuity does not prove the transformer is sound, so follow it with insulation testing and record the readings against earlier results.
Frequently Asked Questions
Q1What is transformer continuity?
An unbroken conductive path through a winding, so current can flow from one lead to the other. It is checked with a low-resistance meter on an isolated, disconnected transformer: each winding should read low, and there should be no path between the primary and the secondary.
Q2Why does the primary winding matter so much?
In a series circuit the primary carries the whole loop current. If it is open, the loop is broken, so the current stops, every fitting on the circuit goes out and the CCR will usually raise an open-circuit alarm.
Q3What happens if only the secondary is open?
The loop keeps running because the primary is intact, but the fitting goes dark. The open secondary can develop a dangerously high voltage and overheat the transformer, so treat the leads as live and find and correct the fault promptly.
Q4What reading should a healthy winding give?
Low and steady, normally a small resistance. Compare it with the manufacturer’s data and the last record rather than a fixed figure, and allow for the resistance of the meter leads.
Q5Why should there be no continuity between primary and secondary?
Because an isolation transformer keeps the two windings electrically separate. A reading between them means the isolation has broken down, so stop and carry out insulation testing before the transformer is used again. An open reading on a continuity meter does not prove the insulation is good; that needs an insulation resistance test.
Q6Does good continuity prove the transformer is healthy?
No. A winding can be unbroken and still have shorted turns or weak insulation. Continuity is one check, so follow it with insulation testing and any other checks the specification requires.
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