24vdc power supply powering instruments/devices

Hello,
I have a question about 24vDc powering instruments from a control panel. My question is why do we use a 2 core cable going to the device/instrument and do not provide a 3rd for for the earth connection like would be provided if the device was powered by AC single phase?

Thanks

All devices must be grounded *, also those powered at 24V, if you use only 2 wires, then you are doing it wrong.

* except for those certified with double insulation with symbol ⧈

Is your control panel isolated? We always used an isolation transformer. The isolated side wasn't grounded except that the 0 volt line might have a 1M Ohm resister tied to it keep the isolated 0 and 24 volts lines from drifting too far.


Grounding the isolated side will induce noise on it. I have seen too many systems where the engineer complained of noise only to find out that the pressure transducer was grounded.

I would be interested to know which devices you have seen that don't have an earth. Answer that and maybe we can answer your question.

But something else for you to think about. Not all AC Single Phase devices have three cores going to them, just the ones that need somewhere for a fault voltage to go to so that it doesn't go through a person. A lot of plastic electric tools are just two core, they are what is called double insulated, so there is no chance of the supply voltage coming in to contact with the operator. If there is a chance of the operator coming in to contact with the supply voltage then an earth is used. It gives a path down to earth and should cause a large fault current to flow, which will then cause the overcurrent protection device to trip. Even better is when an earth fault current detecting breaker is used.

Earthing in 24V d.c. systems is a more complex question, answer my question above to get an answer for that.

Sort of like DeviceNet wiring.

I want to start with I'm not an EE of any kind.
We use DeviceNet for power and communications for our equipment. There's only the + and the - power wires, red/black and the blue/white com lines. There are shield braided wires, but I think in the field the shield wires are not connected to anything or at least to anything meaningful.

Our equipment does have 'grounding' lugs on them, so each device can be connected to an earth ground somewhere in the factory. I am not sure that is done very often or stays connected much pass the initial installation.

People used to connect the 'earth' ground to the water supply or drain pipes, but what starts out as copper or steel can end in plastic or for the drains, end up dangling above a open grate.