Allowing surge arrestor to use the backplate for path to ground

[CHRISC1024]

I have a question regarding surge arrestors for FFB. I would never wire one this way when fabricating a panel but we have some panels that are.
The sure arrestor is internally and electrically connecte to the metal din rail clips attached to the device. The metal clip does also have a screw terminal for a ground wire but the panel builder did not attach one. They contend that it is ok that if there is a lightning strike to allow the surge to go through the arrestor into the panel backplate to get to ground. I am dead set against this as even thoug electricall it would be ok, what damage might it cause to anything else screwed into the backplate. You have PLC chassis and other equiptment screwed to the backplate. I would rather have a ground wire connected to the arrestor to give it a direct path to the ground bar if a strike occurs.

Any thoughts on this. Maybe I'm jsut too cautious when building panels.

[aagostin]

Chris,
What I read from your message is that the surge protectors are using the DIN rail (hat rail, U rail) for conducting the surge current to ground. I would further assume that the DIN rail is then connected via a sufficiently thick grounding wire to the local ground. This would be acceptable, since the DIN rail has a very low ohmic impedance, so that the impact on other equipment is minimal. This was done many times in the past and is working well without problems.
If the DIN rail is simple mounted on the cabinet panel without additional grounding wire, this would not be good practice, as you may not be 100% certain that the DIN rails have full contact to the back panel. The panel may be paint-coated, so that you may only have contact via the screws that fix the DIN rail to the panel. This may not provide sufficient contact.

If you are saying that the surge protector is mounted on a PCB backplane, it depends on the design of the PCB. If this is specifically designed for surge protectors, it will be ok.
An example: let us say you have a PCB that has a dedicated connection for the surge protector ground. The cunductor inside/on the PCB will be separated from other wiring, and the manufacturer will have made sure that no other signals cross the path of this surge ground line. The size on the conductor on the PCB will have sufficient size, so that in case of a surge, the PCB will not go off in smoke.

If the PCB however is made for other type of equipment and then misused, issues may arrive in case of a surge. The high surge current may induce currents into adjacent signal lines, or may even go off in smore if the size of the conductor is insufficient.

Is it possible for you to post a picture of the setup?

[CHRISC1024]

The surge arrestor is mounted on a standard metal hat rail and srewed to the painted backplate. They have added no addtional grounding from the din rail to the ground bar so the only true path to ground would be from te screws ataching the din rail to the painted backplate. This is my concern as there is no direct path to ground and they are relying on th din rail to carry the surge to the backplate and then through the backplate to the ground bar that is attached. I can try to get picture of the setup and post it on here.

I agree, if they bonded the din rail to the ground bar it would be a more acceptable option to me but solely relying on the metal screws as the bodn between the rail and the backplate is to me unacceptable. Hope this illistrates the installation better.

[aagostin]

Since the DIN rail is simply mounted on the panel without grounding wire, this is not good practice. The surge current is diverted into the panel, not to ground. The surge current may choose other paths to ground and damage some equipment, and also the local potential may rise so that some equipment may get damaged. (Let me know if this sentence needs further explanation)

As such, I would strongly recommend to rectify the installation by adding a "ground terminal" to the DIN rail and wiring a ground wire with sufficient diameter to the local ground.

[CHRISC1024]

I totally agree wth you and it makes perfect sense. At a minimum I told them they needed to add a ground terminal to each rail that has a surge arrestor and then brink a ground wire to the gound bus bar. The other option I recommended is to actually add the ground wire to the ground screw on the surge arrestor itself. I have meet much resistance from the panel designer and builder (who are a VERY large automation company). They don't feel there is any electrical issue as-is and are refusing to take any further action. Sometimes I feel that companies that deal striclty with Automation should let people with the elecrical experience take the lead on purely electrical issues.

Thanks for the repsonses Andreas. BTW, I need to talk to Cyrus about a possible site visit as I know he was interested at one point seeing the commision effort and testing that we perform.