Emergency power transfer switch system

1. An automatic transfer switch for transferring the electrical power supplied to a load between a primary power source and a secondary power source, including:

a first relay comprising a SPST bistable relay having one contact connected to a hot lead of the primary power source, and a second contact;

a second relay comprising a SPDT bistable relay having a switch pole connected to a hot lead of the load and first and second switch contacts, said first switch contact of said second relay being connected to said second contact of said first relay, said second switch contact of said second relay being connected to a hot lead of the secondary power source;

a third relay comprising a SPDT bistable relay having a switch pole connected to a neutral lead of the load and first and second contacts, said first switch contact of said third relay being connected to a neutral lead of the primary power source, said second switch contact being connected to a neutral lead of said secondary power source;

said relays being disposable in a non-transfer state in which said first relay is latched to connect said first and second contacts of said first relay to connect said primary power source hot lead to said first switch contact of said second relay, said second relay is disposed to connect said first switch contact of said second relay through said switch pole to said load, and said neutral lead of said primary power source is connected through the first switch contact of said third relay and said switch pole to said neutral lead of said load, whereby a power supply circuit is completed from the primary power source to the load;

wherein said relays may be disposed in a transfer state in which said first relay is unlatched to disconnect said primary power source hot lead from said second relay, said second relay is disposed to connect said second switch contact of said second relay to said hot lead of the secondary power source to connect said hot lead of said secondary power source to the hot lead of the load, and said neutral lead of said secondary power source is connected through said second contact of said third relay and said switch pole to the neutral lead of the load, whereby a power supply circuit is completed from the secondary power source to the load;

further including an electronic controller having internal programming instructions to operate said relays to switch between said transfer state and said non-transfer state;

further including position sensors coupled to said relays for detecting the switch positions of said relays;

further including voltage sensors connected to said primary and secondary power sources for monitoring the voltage and phase of said power sources;

wherein said electronic controller includes programming steps to monitor the voltage of said primary power source on a continual basis, and when the voltage of said primary power source fails, the controller commands the transfer state through the steps:

a) Wait for next zero crossing of primary power source voltage, then open said first relay;

b) Check that said first relay is opened using position sensor signal;

c) Initiate adjustable time delay;

d) Wait for next zero crossing of primary power source voltage then switch said third relay to connect said neutral lead of said secondary power source to said load;

e) Check that said third relay changed position using position sensor signal;

f) initiate adjustable time delay;

g) Wait for next zero crossing of secondary power source voltage, then switch second relay to connect said hot lead of said secondary power source to said hot lead of said load;

h) Check second relay changed position using position sensor signals.

2. The automatic transfer switch of claim 1 , further including a fourth relay operated by said electronic controller and connected to a switch/dimmer lead of lighting fixtures of said load and operated during said transfer state to bring said lighting fixtures to full brightness.

3. The automatic transfer switch of claim 1 , wherein all connections to said third relay include neutral leads from said primary and secondary power sources and said load.

4. An automatic transfer switch for transferring the electrical power supplied to a load between a primary power source and a secondary power source, including:

a first relay comprising a SPST bistable relay having one contact connected to hot lead of the primary power source, and a second contact;

a second relay comprising a SPDT bistable relay having a switch pole connected to a hot lead of the load and first and second switch contacts, said first switch contact of said second relay being connected to said second contact of said first relay, said second switch contact of said second relay being connected to hot lead of the secondary power source;

a third relay comprising a SPDT bistable relay having a switch pole connected to a neutral lead of the load and first and second contacts, said first switch contact of said third relay being connected to a neutral lead of the primary power source, said second switch contact being connected to a neutral lead of said secondary power source;

said relays being disposable in a non-transfer state in which said first relay is latched to connect said first and second contacts of said first relay to connect said primary power source hot lead to said first switch contact of said second relay, said second relay is disposed to connect said first switch contact of said second relay through said switch pole to said load, and said neutral lead of said primary power source is connected through the first switch contact of said third relay and said switch pole to said neutral lead of said load, whereby a power supply circuit is completed from the primary power source to the load;

wherein said relays may be disposed in a transfer state in which said first relay is unlatched to disconnect said primary power source hot lead from said second relay, said second relay is disposed to connect said second switch contact of said second relay to said hot lead of the secondary power source to connect said hot lead of said secondary power source to the hot lead of the load, and said neutral lead of said secondary power source is connected through said second contact of said third relay and said switch pole to the neutral lead of the load, whereby a power supply circuit is completed from the secondary power source to the load;

further including an electronic controller having internal programming instructions to operate said relays to switch between said transfer state and said non-transfer state;

further including position sensors coupled to said relays for detecting the switch positions of said relays;

further including voltage sensors connected to said primary and secondary power sources for monitoring the voltage and phase of said power sources: wherein said electronic controller includes programming steps to switch from the transfer state to the non-transfer state:

a) Wait for next zero crossing of secondary power source voltage, then switch said second relay to disconnect said secondary power source hot lead from said hot lead of said load;

Check that said second relay is opened using position sensor signal;

c) Initiate adjustable time delay;

d) Wait for next zero crossing of primary power source voltage then switch said third relay to connect said neutral lead of said first power source to said load;

e) Check that said third relay changed position using position sensor signal;

f) Initiate adjustable time delay;

g) Wait for next zero crossing of primary power source voltage, then switch said first relay to connect said hot lead of said primary power source to said hot lead of said load;

h) Check second relay changed position using position sensor signals.