Grounding of M for AC500 analog signals


Looking the example of connection diagram below for analog input module AI561 for example, it is shown that the minus of the power supply/module's M terminal have to be connected to the ground.

Is that a must for for minus of the power supply and why so? Is that the protective ground of the cabinet, resp. FE of the controller?

It is strange for me why the power supply potential of 24VDC have to be grounded. Especially when the controller is supplyed from external 220AC/24VDC power supply unit.

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asked 5 months ago



Hi kstoilov

IMHO the connection to the PE ground of the system / cabinet of the shield is one good practice to avoid disturbance of electrical signal in input of the controller/CPU.
However all is depending of the real circumstance of the installation, presence of electrical noise in cables or apparatus / devices with possible irradiated disturbances in the nearest.

Flavio Mussolin   

answered 5 months ago


By kstoilov on 11/3/2017 | Like (0) | Report

Hi Flavio,
Yes, grounding of PE/shields is clear and OK. But in the example is shown that I have to ground the minus of the power supply. Do you have experience with that, because it is not common practice

By kstoilov on 11/3/2017 | Like (0) | Report

I'm asking for connection between terminals 18 and 20

By Flavio Mussolin on 11/3/2017 | Like (0) | Report

Hi kstoilov,
sure, the graphical information is not so clear, but the two pins 18 and 20 are in fact at the same potential: connected to ground and to zero potential of power supply 24VDC.
Why not connect all these points directly to the potential of zero?
I do not have a certain answer to this question ... let's see if any other technician responds
Have a nice day!

By kstoilov on 11/3/2017 | Like (0) | Report

Normally in our systems 24VDC is not connected to the ground (grounded minus). Because in this case, for example for any reason the shield (ground) of external cable is connected to plus (+24VCD) potential, then we will have short circuit on power supply and the PLC will be dead.
Yes, that is interesting to know. I will measure if the M terminal of the modules is internally connected to the PE/FE

By IronGumby on 4/4/2018 | Like (0) | Report

Your last comment is interesting, because in America we do this all the time -- connect an additional reference wire from L- (0V) to Ground. The reason being for "Safety Touch Potential" -- that is, if a hot/energized wire were to come loose and make contact with an enclosure/case. Having the 0V tied to ground will let the fault path travel via the ground to the 0V terminal ("back to the source") and immediately cause the overcurrent protection device (fuse / circuit breaker) to trip/blow. The ABB MTU baseplates are designed to ground itself via it's DIN rail mount, which is what looks like that SG terminal is provided for, since it has a plastic case (and presumably plastic DIN rail mounting clamps? I am not familiar).

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