Monitoring circuit for earth resistance of local mains - has dual transistor network with own DC supply tripping power supply if set value is exceeded

摘要

The earth connection is monitored by a bias supply fed to the neutral and earth conductors through a two transistor network from a d.c. supply. If the earth resistance is sufficiently low the first transistor conducts and the second is biased to the off state. Above a certain critical resistance the first transistor turns off putting the second one into conduction. A relay coil is thus energised and trips the supply circuit breaker. The bias supply can be monitored by a lamp.