I am working on a project to safely combine separate power circuits into one. Its to do with using available power instead of renting mobile generators. The problem is that the 'magic box' has various safety devises that are being hampered by backfeed- as when one supply is cut the eg. contactors remain fed through bacfeed from the other circuits. If it was DC we would , i think be talking diodes, but its ac 230v. I have looked at SSR relays but it seems although no back-feed current will flow there is still a potential voltage present and this is enough to keep the relays/contactors open. I have also looked at using RCD devices to detect some kind of in-balance but the back-feed seems to be balanced through it's own neutral. I have used DP double pole MCB's to provide overcurrent protection but also to offer a higher degree of isolation. I did try running two lives through the DP MCB's and using one feed for the coil. This worked, in that the O.C. devices in the magic box would activate the contactors. But I still have the problem that one of the supplies can be cut outside the magic box with no activation of a contactor.

My Objective is that if one supply is cut off - all supplies will separate.

:Please edit your question to explain your requirements. (1) Are multiple supplies on and connected at the same time? (2) Do you want just one supply on at any time? (3) What is your intended sequence of operation? (4) Is it to be fully automatic?

  1. YES multiple supplies are on and connected at the same time.
  2. NO I do not want just one supply on at any time.
  3. Sequence of operation: 1. Plug in 3 x trailing leads from magic box to 3 x 13amp supply 1ph. Flip trip switches in the magic box to energise 1x 32amp 1ph supply trailing lead from magic box.
  4. Yes once energised the safety should be fully automatic.

enter image description here

Thanks Niall

  • \$\begingroup\$ This question doesn't provide enough info to receive a proper answer. Please add the relevant details in your question, preferably a schematic so that others may understand where the backfeed is coming from. \$\endgroup\$ Jun 4 '16 at 14:22
  • \$\begingroup\$ Please edit your question to explain your requirements. (1) Are multiple supplies on and connected at the same time? (2) Do you want just one supply on at any time? (3) What is your intended sequence of operation? (4) Is it to be fully automatic? \$\endgroup\$
    – Transistor
    Jun 6 '16 at 10:16
  • \$\begingroup\$ You didn't add the information requested in your update? Any particular reason? \$\endgroup\$
    – Transistor
    Jun 6 '16 at 13:45
  • \$\begingroup\$ sorry I didn't see it , all done now. \$\endgroup\$
    – Nile
    Jun 6 '16 at 13:56


simulate this circuit – 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:

  1. 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.
  2. 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.
  3. 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 < 13 A parts.

Provide more details of the application and help will be forthcoming.

  • \$\begingroup\$ @Nile: See the update. \$\endgroup\$
    – Transistor
    Jun 6 '16 at 14:21
  • \$\begingroup\$ Transistor you are stating the obvious negatives and struggling to understand a clear statement I would prefer you not comment further. The objective had been stated as finding a safe method. 230v AC inherently dangerous in all situations without adequate safety precautions. \$\endgroup\$
    – Nile
    Jun 7 '16 at 21:01
  • \$\begingroup\$ Not quite, Nile. The original question was not clear. You clarified it after my OP comment - see the edit history. You have a rep of 1 on the site so I can't make any assumptions about what you know so I spelled it out. Bye. \$\endgroup\$
    – Transistor
    Jun 7 '16 at 21:26
  • \$\begingroup\$ First sentence ' I am working on a project to safely combine seperate power circuits into one' no edit. \$\endgroup\$
    – Nile
    Jun 8 '16 at 21:35

If you are trying to do what I think you are trying then this is actually a REALLY hairy problem, and is almost impossible to do in a way that is both economical and reliably safe under all the possible failure modes.

There are basically two approaches:

1) Convert all three inputs to charge a high voltage DC link cap then convert the DC back to AC, basically three active PFC networks and an inverter, the PFC networks will probably need to be isolating (This has a relatively sane safety case, but is expensive).

2) Actively sense the direction of power flow in each circuit (Basically integrate V*I over a cycle for each circuit) and cut off any that go negative. You would also need a large common mode choke on at least two of the circuits to ensure the neutral currents balanced, the safety case for this is a nightmare but conceptually it could be made to work.

This sort of thing is actually much easier with three phase generators as you basically set the machines off load voltages to be the same and then arrange for the three machines speed regulators to droop by the same percentage at full load, this sets up automatic load sharing, but you must make sure the machines are in sync before bringing them on line and watch for excessive circulating VAr, usually means you got the off load voltage trimming slightly off)...

Personally, I would just hire an appropriate generator or pair if I needed the redundancy, easier, cheaper and more reliable then trying to build a safe way to do this.

Regards, Dan.

  • \$\begingroup\$ Hi Thanks for your comments Dan, I will se if I can add a drawing of where I am so far but your right it seems way more complex than I had hoped. \$\endgroup\$
    – Nile
    Jun 6 '16 at 8:30
  • \$\begingroup\$ By the way Dan what device might intergrate V*I over a circuit. I was looking at RCD's just to act as a balance sensor on each circuit, but again the backfeed seems to balance. \$\endgroup\$
    – Nile
    Jun 6 '16 at 8:33
  • \$\begingroup\$ You could do the multiplication with an analogue multiplier followed by an opamp integrator, or more reasonably by means of a small micro with built in AD converters. Note that this is safety critical stuff, so some paranoia is advised. \$\endgroup\$
    – Dan Mills
    Jun 6 '16 at 19:11

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