6.5 Hi-pot AC Test (HPAC)
Transformers may be used to provide isolation between dangerous line input voltages and safe low voltage outputs. The transformers may be separate line frequency transformers or power or control transformers inside power supplies and similar equipment. To ensure user safety throughout the life of the transformer the critical insulation must be tested at a very high voltage. No breakdown may occur.
A Hi-POT (High Potential), or ‘flash’ test is applied to isolating transformers to guarantee the integrity of safety critical insulation in accordance with international standards. Primary to secondary insulation is always tested, primary to core or ground or primary to core + secondaries or core to secondaries may also be specified. Standards often describe Hi-Pot tests at ac voltages; a dc test at a voltage equivalent to the ac peak may be acceptable.