To start emergency generators, transfer switches typically use a signal circuit controlled by a normally closed contact. When utility power fails, the contact de-energises and closes and a start signal is sent to the engine, as shown below.
Monitoring Engine Start Signal Circuits
A problem occurs when the generator control wiring is cut or malfunctions, resulting in the open fault shown below. This could result from modifications to the normal or emergency power systems, or when damage occurs such as accidental cutting of a conduit or wire. When open faults are present, the engine will not receive a start signal when utility power fails. The engine controller would still see an open circuit, and would not start the engine when needed.
A short could also develop in a system that uses a normally open contact (which is held closed when generator power is unnecessary). Again, the engine would not start even when normal utility power became unavailable. Because of susceptibility to this type of fault, an engine start circuit equipped with a normally open contact would not fully comply with NEC requirements. The following figure shows a short occurring along a circuit equipped with a normally open contact.
To avoid risks posed by inoperable start signal circuits, the Article 700.10 of the 2017 National Electrical Code® requires (1) continuous monitoring of generator signalling circuits, (2) visual and audible annunciation of changes in state, and (3) automatic transmission of an engine start signal and alarm when a problem is detected. ASCO’s 5101 Engine Start Modules address all three elements of the new code.
Learn more about the benefits of engine start circuit monitoring by review our white paper entitled Monitoring the Integrity of Engine Start Signal Circuits. Learn more about the ASCO's engine start solution by viewing the ASCO Model 5101 data sheet.