I`m designing a control cabinet with a 60A output demand for heaters. The on/off switching of the heater will be done through a 24V/80A contactor(relay). The customer wants the main switch on the door. That requires 2x5x10mm2 wires to and from the door switch. Unfortunately, there is no space for the switch on the DIN rail with a long stick to the door. I offered 2 solutions: 1. Place the main switch on the side, not on the door. 2. The main switch operates only L1 for the control electronics and L2, L3 going strait to the MCB, and then to the contactor(relay), so I do not need to switch the 60A on the door, only the small current for the control electronics. Is this legal to do this way? My question is, which solution is the right one? Or if none, then how would you do it? Thanks!

  • 1
    \$\begingroup\$ "Is this legal" very much depends on where this is going and the application, so we can't answer that. You could also use a smaller relay to control the bigger one so that you only have to run enough current to drive the coil through the switch in the door. \$\endgroup\$
    – Ron Beyer
    Jun 5, 2020 at 15:28
  • \$\begingroup\$ Hi Ron, thank you for your reply! My concern is mainly the following in case 2.: The door switch operates as an isolator, however, it is not switching all the incoming 3 phases. It switches L1, that gives power to the 24V PSU and that powers all the control electronics, which can operate the 80A contactor, when it is needed. L2 and L3 goes directly to the MCB and from there they are going to the contactor. The customer seeing this as a product, so it could go anywhere. Thanks! \$\endgroup\$
    – TheSaint
    Jun 6, 2020 at 16:45

1 Answer 1


From the safety perspective, it would be logical to disconnect all three phases with the main switch. Maintenance personnel would also expect/assume that it would be so.

Door-mounting of the main switch would only be an alternative to side-mounting without any added benefits. In fact, door-mounting would be more unsafe than side-mounting, considering the routing of the 10mm² cables with sufficient slack and the chances of exposure of personnel to live points.

A door-interlocked main switch would render the cabinet inherently safe and would also conform to safety norms worldwide (considering it as a product and not a one-off).

In my opinion, a door-interlocked main switch, to disconnect all three phases, would be the right choice.

It would be worth your while to redo the layout.

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    \$\begingroup\$ Agreed. The main isolator must remove all power to the panel. \$\endgroup\$
    – Transistor
    Jun 7, 2020 at 9:09
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    \$\begingroup\$ Thank you for you all! The door-interlocked mains-switch will be then... Thanks and have a good day! \$\endgroup\$
    – TheSaint
    Jun 8, 2020 at 10:31
  • \$\begingroup\$ Anytime, TheSaint! \$\endgroup\$
    – vu2nan
    Jun 8, 2020 at 11:07
  • \$\begingroup\$ I would be glad if you would accept my answer. \$\endgroup\$
    – vu2nan
    Jun 26, 2020 at 6:22

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