LEGACY ELECTRICAL NON COMPLIANCE ISSUES 2

[randman2]

Recently our company had moved a piece of equipment from one location to another location, while installing and troubleshooting the electrical system I discovered what seem to be electrical safety non compliances. As a matter of ethics I believe the compliance issues should be fixed.

The control circuit for the MCR relays is 24VDC, the EPO loop back to the relays is run with the -24DC side of the ungrounded 24VDC power supply, in this case the power supplies all float, (no common ground from the -24VDC to earth), normally the -24VDC system supply is common grounded to earth and the EPO loop is +24VDC. This machine was built 9 years ago,the standards for safety controls were in place at that time still prevail, also there is no redundancy in either the EPO loop or in the MCR circuit.

This machine I know is at least a category 3 level machine,(it can cut fingers off). I have disclosed this to my supervisor and he has chosen to ignore it. What are the implications of letting this go, and is this somehow an accepted methed for an EPO circuit 9 years ago.

 

[cachehiker]

I believe these circuits are generally grounded for electromagnetic compatibility (either immunity or emissions), to prevent shock or fire hazards, or to protect other equipment that might be connected to them. I don't have the standards in front of me but we weren't required to ground our 24V supplies when I was doing automation work unless it was to protect something. I now do quantitative analysis but am occasionally required to help out with compliance issues.

Would the current circuit continue to function when subjected to radiated and conducted emissions, line surges and sags, or ESD pulses? Is there any reason why either supply rail of this 24V supply would present a shock hazard? It would either have to be accessible or capable of energizing the machine chassis to shock someone. Is there enough power available from it to constitute a fire hazard under any load? Is there overload protection in the form of fusing or other means?

 

[itmBen]

Hi Randman2,

The fact that the EPO circuit is not grounded should not effect the compliance of the system if the system has been designed this way. (if the ground has lifted then it is a circuit fault which should be fixed accordingly). Be aware, some circuits rely on a floating supply in order to detect faults.

 

[randman2]

Thank you cachehiker itmben for your reply and good information. In response to your reply I still have an argument for grounding the -24DC to earth ground. Some manufactures of third party devices that run on system 24VDC like (STI Light curtain controllers) of a few years ago grounded the -24 side of their controllers to chassis ground and by default the -24VDC was taken to earth ground 50ft away from the power supply, this grounding caused ground loop problems, if your going to earth ground the (-)24DC then you do at the power supply and as close and short as possible to the incoming PE. I guess I have not been around different types of control gear enough because I have always seen this done by the big industrial CNC manufactures dating back to the early 90s, example Union Carbide CNC plasma cutters grounded every power supply (-) to earth ground this included the 5VDC logic supplies the 24DC system control supplies. Westinghouse CNC beam line systems followed the same conventions. I believe there are noise immunity benefits to this convention as well as safety. My problem is I don't believe the requisite level of safety for the type of equipment I described earlier is sufficient enough to meet category 3.