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(I know, relay are drawn in the wrong way, NO contacts should be in serie with load, I'll fix it ASAP)

When I have to build a board to switch many devices (mains powered), I always use a single "common rail" for all loads, like the one in this image:
Schema1

But in the past few days I had to fix two fridge (two different brands) and I noticed that loads connected to the control board relays used a rather different schematic:
Schema2

I wonder why they've used this type of connections. I add FWIW that there were no particularly heavy loads, and wires, relay and PCB traces were all correctly sized for the power needed, and moreover there were no analogies between "grouped" devices.

(I know, relay are drawn in the wrong way, NO contacts should be in serie with load, I'll fix it ASAP)

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  • \$\begingroup\$ Do the two groups have separate fuses? If one fuse blows, only half of the attached devices will stop working. \$\endgroup\$ – jippie Aug 10 '12 at 10:08
  • \$\begingroup\$ No. Single fuse (and placed directly on the mains cable). And to confond me more in one of the fridges the two compressors were both in the same group, in the other they were "separated" \$\endgroup\$ – Axeman Aug 10 '12 at 10:14
  • \$\begingroup\$ Colors. Fix them, please. \$\endgroup\$ – Christoph B Sep 9 '12 at 12:28
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There's little difference between the two circuits: in the lower one the switching occurs on the live side for three loads, and on the neutral side for the other three. For the operation it makes no difference: power is cut off when the relay opens.

There's a safety issue, though. Switching the live wire would make the power connections on the load more safe to touch. Ideally the neutral should carry zero voltage, in practice a low voltage is possible.

I said more safe, not completely safe, though. Cutting off the live wire may give you a false sense of safety, but the relay contacts don't have the required separation for absolute isolation. A relay contact which erroneously closes is normally not seen as a serious problem, as it's meant to close anyway. That's why it only has a small separation distance.

So, even if the live wire is switched, never work on the load's connections without cutting power by switching off the automatic fuses for both live and neutral.

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  • \$\begingroup\$ Agreed on the safety issue. But my curiosity is really about that little difference that's in your first sentence. I mean, the second circuit is a little more complex, and you have to route both live and neutral to relay board and to the loads. And it's true that there's no difference in the operation. So, there are advantages in using the second circuit? \$\endgroup\$ – Axeman Aug 10 '12 at 11:54
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If relay is connected to Line, Load must be between switched output and neutral. If relay is connected to Neutral, load must be between switched output and Line.

Direct connection to line is only allowed on UL/CE approved special circumstances with proper protection from human contact with any test including HIPOT. Otherwise it is an unsafe design.

Is the fridge repair relevant to this question, such as an improperly rated relay current for the surge current? If so the surge current must not cause a significant voltage drop in the relay contacts and have sufficient insulation arc suppression filter to prevent flashover on opening the contact that may contribute to motor failure if there is a centripetal starting current boost when stalled and meanwhile the relay is still arcing from flashover and insufficent rating and/or lack of snubber filter.

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  • \$\begingroup\$ The fridges failures were both related to the controller digital logic. In both of them, anyway, relay commutation were zero-cross synchronized and I never saw any sign of sparkling from the contacts (cases were transparent) But that's not related to my question that is in brief "There are advantages in using the second circuit?" \$\endgroup\$ – Axeman Aug 10 '12 at 12:00

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