| 摘要 |
<p>An earth leakage protection device comprises a core balance current sensing transformer having a secondary winding in which a fault signal is generated in response to leakage from AC supply lines forming primary windings of the transformer, an operational amplifier for amplifying the fault signal, a precision rectifier circuit comprising a pair of operational amplifiers for performing full wave rectification of the amplified fault signal, delay circuit in the form of a two stage RC network which delays the rectified signal according to an IDMT characteristic, and a trigger circuit in which the delayed signal is compared with a preset reference signal by an operational amplifier to provide a control signal which actuates a safety device in the AC supply lines via a relay when the reference signal is exceeded. Within its operating range the device actuates the safety device much more slowly for lower leakage currents than for higher currents, and since it is operative on both halves of the AC leakage current signal the device provides reliable protection against the possibility of dangerous electric shocks while avoiding spurious tripping.</p> |
