ce regs secondasecondary earth

[Christopher]

Guys

What is the best practice with earthing?

I have always been told to take every point which needs earthing (metalwork, earth's of psus.  What else??) Back to one star.

But sometimes this is fairly impossible. There are only so many rung tags I can fit on a stud! A secondary earth point with a low impedance 2.5mm cable back to the main earth point.

Any bs en specs on this or any good safety manuals on this?

My main earth normally comes from an iec socket and a dc or ac source in parallel so we always have an earth availableavailable

[Neilm]

In the past I have just ensured that there was a low impedance high integrity path to Earth. What standard are you trying to meet?

[German_EE]

Theory says that every metal panel on a mains powered device should have an earth connection. In practice eight or ten screws securing a large metal panel to a grounded chassis with paint removed inside to ensure a good electrical contact mean that a ground strap can be avoided. This is especially true if all mains wiring is secured and it is physically impossible for a live wire to touch a panel.

Think about Dave's teardowns (of the good stuff). You will see a ground connection to chassis and possibly the bottom panel but the top piece of the case will have no ground wire.

Go to a German train station and you will see that even the metal seats and the rebar in the concrete is earthed, some people take things too far.

[Tandy]

Go to a German train station and you will see that even the metal seats and the rebar in the concrete is earthed, some people take things too far.

That will be bonding rather than earthing.

The purpose of the earth connection on a chassis is to create a low impedance path to earth to blow a fuse. Bonding ensures that everything is at the same potential. If you sat on a metal seat that was at a high potential and then touched a window that was connected to earth you would create a circuit between the two and the charge would go through you. If however the two items were connected with a bonding wire they would both be at the same potential so you would not create a circuit.

[DenzilPenberthy]

At the risk of venturing off topic: Electric railways have some fairly stringent requirements for earthing and bonding. They're not just doing it because they like spending money on copper. This is due to very large earth currents flowing from the rails which are used as a ground to return the traction current.  When you have very large currents flowing in the ground you have to be very careful to bond everything together to limit potential differences between objects which ought to be all at ground potential.

[SeanB]

Correct, consider the typical traction current is in the order of 1kA, and then think of a station made from steel structure with a ground connection at the one end, and it is 100m long so a train can stop there with all cars under cover. With 1R resistance on the tracks in the station there is a 1kV drop along the length of the station. Think if the structure was only bonded at the far end, and you were to incidentally touch the rail or the stopped train while simultaneously you were holding on to the steel structure. Even if there was 0R1 resistance the voltage would still be over 100V. Thus a lot of bonding conductors, all of which have to be capable of handling the full traction current of the line.

Even with rail cars which are around 12 tons empty, and a traction unit of around 40 tons a train is still capable of pretty good acceleration, limited only by slip of the tyres on the rail. That does need some serious power from the overhead line and this travels through the rails.