HMI and NEC Electrical Safety

[David.Reed.Schmidt]

Greetings,

I have a coworker who is upgrading an electrical panel with a RedLion Graphite series HMI with I/O cards and RS485 outputs in the back to control 4 motor drives and other devices from the panel.

He and I were discussing how to incorperate elecrical safety into the panel. He would like to install the main disconnect device on the door of the panel which is great. He's planning on cutting all the power to the interior of the panel to include the HMI.

Can anybody share thier thoughts regarding isolating power to the panel except for the HMI. Any thoughts regarding any other low voltage devices.

 

[danatomega]

There's no need to cut the power to the HMI especially if it's low voltage.

 

[David.Reed.Schmidt]

Is there some code guidence that governs electrical safety for low voltage items inside panels?

If I knew the locations of the code that dictates these rules than I'd be better able to interpret them beyond the HMI. Perhaps if we had an ethernet router inside the cabinet. Surely no need to cut power to that.

Also, do finger safe connectors (IP 20?) can mitigate the chances of a person getting shocked by 24v connector.

I feel there is no need to cut power. Afterall, it's low voltage. And since the HMI is the brains of the panel there might be additional safety factors that come up when somebody bypasses the safety disconnect in order to power the HMI to do some additional trouble shooting. A situation I'd like to avoid.

 

[danatomega]

I don't have the exact number but I believe somewhere about 48 Volts DC and below is considered non-hazardous. But I'd have to check some regulations to be certain. NEC (National Electric Code) would have the values.

 

[blackandredwarrior]

You can bring a separate feeder specifically for the HMI (say 120VAC to feed a power supply). You need to specifically color code that wire to note that it is still hot while even with the disconnect off. Then label the panel that power may still be present and we usually note where this power comes from.