Multicore cable earth

We have a local push button station going to a PLC in a safe area. There is a 7 core cable connecting the station to the plc.

Core 1 24v
Core 2 0v
Core 3 push button 1
Core 4 push button 2
Core 5 spare
Core 6 spare
Core 7 earth

My question is why is core 7 an earth cable, can’t think what I’m missing?

Thank you
It’s a NO push button. Also there is no shielding on the cable. The box is earthed to nearby steelwork. I was thinking the earth cable was to stop induced voltages in the other cores?


Is the earth core terminated to earth at one end or both ends?

i have seen cases where one end of every spare conductor is terminated to earth, austensibly for personnel safety reasons. That’s taking safety to a pretty extreme level—in my personal experience. But that was site practice and “when in Rome ...”

It sounds as if the “designe” may have been trying to protect against induced voltages, but the proper solution for that problem is to use a shielded cable (overall cable shield) with a drain wire and earth the drain wire at one end.

If the cable doesn’t have an overall shield and drain wire and it needs to be protected against induced voltages because of how/where the cable is being routed, then the choice of cable is incorrect.

But, again, some sites/plants have “unusual” requirements, and if that’s the case the you should follow their requirements. Unless you feel that in this instance there is a particular reason or problem with the requirement which may be unsafe or cause problems.

Hope this helps!
Thank you for this. As this is a push button station with simple NO push buttons I believe that cored cable instead of twisted pairs is a suitable choice. The client has no specific cable specs that I have seen. I was just curious that if the enclosure was earthed to steelwork then why the cabling also contained an earth core.

Thanks again.
The industries I have worked in make an assumption, whether the enclosure was outdoors or not, that bonding to steelwork would start to corrode within a few years. Indeed I’ve seen some onshore installlations where steelwork corrosion is extensive. Running earth cable to an earth terminal which directly contacts the terminal rail is common practice.

Why core 7 ? In a 27 core cable it would be core 27.....just out of the way of power/signal cores but still there.