How do I prevent backfeed through contactors when joining 3 x 230v ac circuits all single / same phase
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Multiple generator interlocks.
An arrangement such as Figure 1 would prevent two supplies being connected simultaneously. The first relay / contactor to energise will prevent the other two from energising. Many contactors allow addition of auxiliary contact blocks for additional contacts. If you can't do this then add a signal relay in parallel with each contactor and use the contacts of those to interlock the contactors.



simulate this circuit
Figure 2. An even simpler arrangement but this suffers the problem of momentary interconnection between the supplies.
Consider the case in Figure 2 when GEN2 is on and GEN1 is restored. GEN1 contactor will energise and this will cause CR-2 to drop out. During this switching sequence both outputs will be connected for a short time. If they are out of phase high currents will flow. This option is not recommended.

Update after additional information supplied:

Sequence of operation: 1. Plug in 3 x trailing leads from magic box to 3 x 13 amp supply 1 ph. Flip trip switches in the magic box to energise 1 x 32 amp 1 ph supply trailing lead from magic box.

At a guess I'd say you're trying to get a single 32 A supply in a building that only has 13 A sockets. There are several problems:

With the backfeeds (and the risk of backfeeds if you come up with a solution) the 13 A plug pins will be live if unplugged. This is potentially lethal and a good enough reason to stop now.
If you use the system in a building with 3-phase supply and sockets on different phases you risk shorting two phases. You might not get caught out
but someone else may. Another good reason to stop now.
You have no guarantee that the currents on each feed and return will be balanced. i.e., You could have current coming out on the live and some returning via another neutral. If there were independent RCDs they would trip and there could be confusion regarding the cause. Another good reason ...

I suspect that you would be in breach of some regulations with this setup. My approach would be to split the load, if at all possible into &lt; 13 A parts.
Provide more details of the application and help will be forthcoming.