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Given a power supply in an electrical enclosure powered by 120 VAC. An I/O device will trigger a relay with a 24 VDC control. The signal from the I/O device will enable the relay for a set duration. The user wants the relay to switch 120 VAC to power an eject device.

What would be the considerations given to determine if the 120 VAC connected to the relay output need to be isolated from the primary input power?

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    \$\begingroup\$ The enclosure is not powered, the things inside it are. The relay does not supply power, it just switches it. Whether it needs to be an isolated supply depends on your application. \$\endgroup\$ – HL-SDK Mar 31 '14 at 17:27
  • \$\begingroup\$ @HL-SDK Fair enough. I've modified the question. \$\endgroup\$ – Rich Shealer Mar 31 '14 at 17:33
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Used the way you describe, a relay does not provide isolation between the mains and the device being powered.

It does provide isolation between whatever is providing the control signal (to open and close the relay) and the power circuit (assuming there are no other connections between them).

You can isolate the powered device from the mains using a transformer, if that is part of your system requirements.

What would be the considerations given to determine if the 120 VAC connected to the relay output need to be isolated from the primary input power?

If the device being powered does not have any specific safety features, it would be a good idea to isolate its power from ground. The reason for isolation is that the neutral wire of mains is tied at some point to earth ground. If the user contacts the hot wire (or some part that is connected to hot by a fault) a hazardous return path could be made through the user's body. Safety isolation breaks this path.

However the load device might already be designed with its own safety features (such as isolation or double-walled shielding) to allow it to be powered from mains, and in that case you wouldn't necessarily have to provide isolation in your switching device. You would want to make sure the cable between the switch and the load is routed with due regard to safety (for example, through earthed conduit).

Also remember that give the supply voltage is 120 V, isolation alone is not enough to ensure safety.

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  • \$\begingroup\$ The question only said that power was being switched, but not where the power came from. The device being switched is at the moment unknown other than it requires 120 VAC to be activated. It may vary from line to line or location to location. \$\endgroup\$ – Rich Shealer Mar 31 '14 at 19:22
  • \$\begingroup\$ Did you mean does not offer safety features? "If the device being powered does safety features, it would be a good idea to isolate its power from ground." \$\endgroup\$ – Rich Shealer Mar 31 '14 at 19:23
  • \$\begingroup\$ @RichShealer, if the device might sometimes be powered from mains, then it should be designed to be safe when powered from mains. If it might sometimes be powered from some other 120 VAC source, then it should be designed to be safe under those conditions too. If you're running this thing on an airplane or a ship or something, you're beyond my area of knowledge. \$\endgroup\$ – The Photon Mar 31 '14 at 20:25
  • \$\begingroup\$ I suspect it is some sort of coil that actuates an arm that removes product from a manufacturing line. I was just looking for a best practice if I should ask for an isolation transformer or not. \$\endgroup\$ – Rich Shealer Apr 1 '14 at 2:08

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