The plan is to fit a suitably rated double pole contactor which is capable of connecting the 13 amp sockets either to the output of the inverter or, when available, to a mains hook-up.  By connecting the energising coil of the contactor to the incoming mains supply the contactor can be made to automatically change the sockets over to 'hook-up' when the mains is applied.




 We currently have our inverter in a cupboard and anything that needs AC power is plugged directly into it.  Simple, not always convenient and a pain when more than one appliance needs AC power. So this task is to let the inverter feed ALL the van sockets when the real mains is not present.

 'The first job was to install a remote switch for the inverter so that one  does not have to reach to the back of a cupboard to switch the inverter on and off each time.  (Never leave an inverter on when not in use as, even in standby mode, as it draws a small but significant current  from your battery.)'.

            Both my colleagueís and our own  motor home have very similar mains electrical circuits. The mains enters the vehicle via a blue 16 amp connector and is routed to a 'Consumer Unit'. The Consumer unit consists of three components.

1)                  A double pole residual current circuit breaker (RCD) this also has an overload sensing feature set at 40 amps.

2)                  A double pole isolator rated at 6 amps. This isolator feeds the battery charger, mains part of the fridge, water heater and interior mains voltage lighting.

3)                  A double pole isolator rated at 10 amps. This feeds all the 13 amp mains sockets in the motor home. 

Unlike a full domestic arrangement which protects sockets connected via a 'ring main' with a 30 amp isolator the 10 amp limitation of the isolator fitted in the motor home means itís quite safe to connect a chain of 13 amp sockets one after another. Just donít try a 3 kilowatt kettle else the isolator will switch off! 



The  contactor has 4 sets of 'contacts' each rated adequately for voltage and current.  It is rated at  16 amperes  and has a 230 volt AC coil, its contacts are rated for 415 volts to suit 3 phase applications. Its manufacturers reference is  A-B 700-CF220 and it is available as a stock item from R.S.Components as  Part number 369-1952

 Two sets of contacts close when mains is applied to the coil. These contacts are used to connect the mains to the sockets when the incoming mains is connected to the coil which  causes these 'normally open' contacts to close.

The remaining two sets of contacts are used to connect the inverter output to the sockets. These 'normally closed'  contacts will open when mains is applied to the coil thus disconnecting the supply from the inverter.

Having emptied the wardrobe access was gained  to the consumer unit and battery charger  in the motor home . To the right was a suitable space and a plastic electrical enclosure was positioned to accept the new 'change over' contactor.

            Having ensured that the motor home was NOT connected to the mains hook-up I commenced work. Covers were removed. Inspection showed that all the mains wiring to the sockets was 2.5 mmsq flexible  double insulated cable, not the stiff flat cable used to wire houses. Also only one pair of wires went to the output of the 10 amp isolator, the first socket in the system acted as a distribution point for the other sockets. All I needed to do was to disconnect these two wires from the 10 amp isolator and connect them to the  one side of both the normally open and normally closed contacts.  To make this easier  I pre-wired the contactor  and joined the pair of tails to the wires removed from the isolator with a suitably rated screw terminal block. The conductors were stripped back such that both pinch screws clamped both wires. This terminal block  is  protected by the consumer unit cover.

The remaining normally open contacts were connected in parallel with the contactor coil terminations and also to the output terminals of the isolator. The inverter was connected to the remaining contactor normally closed contacts.




Finally I added one more feature. I decided to switch OFF the inverter automatically should it be running when the mains hook-up was connected. This was done with a smaller relay with a mains voltage coil.  This relay had  several sets of contacts and I used a contact set that were 'normally closed'. These contacts were later connected in series with the remote ON/OFF switch  for the inverter slave relay. All the mains wiring was done using 2.5 mm sq  mains grade PVC insulated wire which  was routed through plastic  sleeving between the contactor enclosure and the consumer unit. 

The final arrangement of the contactor box  after wiring  was completed and covers re-fitted is shown above.

Obviously this arrangement has some limitation governed by the size of the inverter and the size of the leisure battery. This limits the size of the appliances that can be plugged in. But it now means that the TV, laptop, Phone charger, Camera charger and even  the most important small food whisk can be used from any socket outlet in the van.



New regulations now preclude amateurs from  doing mains wiring in their own  houses, but so far this legislation is not believed to extend to  motor homes.  This MAINS WIRING  is potentially lethal if not done correctly. Several  details are omitted from this description and if you are qualified to do this work  you will not need to ask.  If you are not qualified to do this task then employ someone who is.


One of the more conspicuous mains sockets was fitted with a Neon indicator to remind us to switch off the inverter when it is not required.




Return to home page