Miniature Circuit Breakers W T henley Miniature Circuit Breakers have a low rate of impedance and an extremely low watt loss making them one of the most energy efficient MCB’s available. Our range offers type B tripping characteristics for domestic or commercial installations such as light bulbs and heaters. Manufactured from Polybutylene Teraphtalate, W T Henley Circuit breakers use an electromagnet, which responds instantaneously to short circuit current, together with a bi-metallic strip that responds to less extreme but longer-term over-current conditions.