XPost: sci.electronics.design   
   From: tw@dionic.net   
      
   Jon    
     wibbled on Monday 08 March 2010 13:53   
      
   > The electric service goes to the house, but I want to run the generator in   
   > the barn.  So far I know,   
   >   
   > * there are 3 unused #10 wires buried between the house and the barn.   
   > * the electric to the barn comes from the house in 2 buried #000 cables.   
   >   
   > Some of the parts I have to work with are   
   >   
   > * a 3pst relay with a 120vac coil and contacts rated for 90amps   
   > * the above coil also activates 4 N.O. and 4 N.C. contacts rated for   
   > 15amps * two 3pdt relays with a 120vac coil and contacts rated for 30amps   
   > at 300vac * a spst mercury relay with a 240vac coil and contacts rated for   
   > 60amps   
      
   To switch the entire house over, you need a changeover switch or relay rated   
   to full utility supply current (ie whatever your utility protective fuses   
   are rated at). The key word is changeover - nothing else will do.   
      
   You do not appear to have that device in your inventory.   
      
   The other solution is to split your house circuits into 2 groups, essential   
   and non essential and put the wiring to each on separate breaker panels. Use   
   the 30A DT relays to switch the supply to the *essential* breaker panel   
   between utility and generator.   
      
   This ASSUMES the relay in question has the required isolation parameters and   
   is GUARANTEED break before make.   
      
   Also, you would probably feed the utility supply to the "essential" breaker   
   panel from a 30A breaker off the main panel to provide adequate protection   
   to the relay and wiring which constitutes the distribution circuit.   
      
   And you may need to sort the grounds out in the event of local supply.   
      
   What I've described would more or less fit with the British IEE Wiring regs,   
   but would need cross checking in detail with the NEC and any local codes.   
      
   But there is a lot riding on the transfer device specifications due to the   
   fault currents it may be expected to handle (100's of amps at least - here   
   my potential short circuit current is 1200A at 240V, supply impedance having   
   been measured at around 0.2 Ohm) and the isolation characteristics - even I   
   would be unhappy about putting an unapproved part to that use.   
      
   Would a say, 500A surge weld your contacts before the main fuses clear the   
   fault?   
      
   Do not even think about using your 3pst relay to isolate the utility   
   incomer, that would be against the rules here with good reason and I would   
   wager it is against yours.   
      
   Here it considered quite normal to split the wiring into 2 groups as most   
   home generators would never supply a house under full load and are used just   
   to keep the gas/oil heating going and a few lights.   
      
   However, it probably won't be any cheaper to buy and install a second panel   
   than it would be to buy a type approved transfer device and sleep easy at   
   night.   
      
   --   
   Tim Watts   
      
   Managers, politicians and environmentalists: Nature's carbon buffer.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)