Can I safely run my boat generator while connected to shore power?
My boat has a built-in diesel generator and a Shore-Off-Generator selector beside the AC panel. Can I run the generator for its monthly exercise with the marina cable still plugged in, and should I switch off the shore supply first?
Answer
Yes, you can usually run a permanently installed marine generator with the shore cable plugged in, provided a correctly installed transfer system prevents both supplies feeding the same circuit together. Check the source selector and the boat's operating instructions first: a plugged-in cable does not necessarily mean the shore supply is electrically connected.
Check supply separation
Look for a Shore/Off/Generator selector, mechanically interlocked breakers, or an automatic transfer switch. An ordinary manual selector should be break-before-make: it disconnects one source before connecting the other.
Two separate breakers labelled Shore and Generator do not prove there is an interlock. If both can be closed together, have a marine electrician establish how the system prevents the sources connecting.
Connecting an ordinary generator directly to live shore power can damage the alternator, start a fire and backfeed electricity towards the pontoon. Never connect a generator through a socket outlet or a lead with exposed male pins.
Some larger yachts have separate AC buses or purpose-built synchronising equipment. These may allow simultaneous operation, but only in the arrangements specified by their designer. A conventional selector cannot do this.
Follow the changeover sequence
Many installations let you run the generator with its output breaker open while shore power continues to supply the AC panel. Check the manual before doing this: an automatic transfer system may change sources as soon as generator voltage appears.
For a typical manual installation, follow this sequence, adjusting the generator output-breaker order to match the manufacturer's instructions:
- Switch off heavy AC loads, including air conditioning, the immersion heater, cooker and battery charger.
- Turn off the boat's shore-input breaker and move the source selector to Off. The cable can remain plugged in if the installation permits it.
- Check oil, coolant, the raw-water seacock and strainer, then start the generator with its output unloaded.
- Confirm cooling-water discharge and allow the specified warm-up period. Check that voltage and frequency settle at the boat's rated values, commonly 230 V and 50 Hz on UK boats.
- Select Generator and enable its output as instructed, then restore loads individually.
To return to shore power, remove heavy loads and disconnect the generator supply through the controls. Select shore through Off, then restore loads. Give the generator its specified unloaded cool-down before stopping.