I think we've been the victims of history a bit on this one.

When I first saw a decent sized computer installed (back in the late 1980s or early 1990s), it had a 'clean earth system' - i.e. it's c.p.c and and additional 10mm² G/Y went directly back to the MET and connections with other c.p.c.s in the area (e.g. air conditioning) were avoided. The idea was to try and segregate it from all other equipment that might somehow introduce "interference" onto the earthing system. (If it wasn't for the fundamental requirements of EEBADS I dare say some would have wanted an independent electrode to connect the computer to).

The problem was that it didn't really work as intended - the main computer might have had a nice clean earth, but it was connected by (earth referenced) RS-232 cable to terminals (as in CRT teletypes, rather than bit of brass to join wires) scattered all around the building, which in turn were just plugged into the local ring circuits. So not only did the clean earth get polluted by all the other connections, the 0V lines in the data cables themselves could end up carrying unwanted current, so possibly even making the situation worse.

So these days a completely different approach is taken - rather than try to keep different "earths" apart, have just one and make it a low impedance as possible by having as many parallel paths as possible. Hence all the talk of meshes and rings and stars of meshes in BS 7671 and BS EN 50310. Some of the old language about separate "clean" earths does seem to cling on in some quarters though.

Likewise with METs. If you'd asked a few years ago, I think the general consensus of opinion was that you had one MET per installation, and if an installation spanned  two more buildings, the subsidiary buildings had earth marshalling terminals (BEMTs). More recently the BS 7671 requirements for main bonding have changed, reworded, and are now very much more clearly 'per building' (and only internal to a building too), rather than per installation (and applied to each building) - which in a way makes things rather simpler, and the naming each building's earth terminal a MET makes more sense. But again much of the older language persists in many texts (and likely will continue to do for a while yet).

   - Andy.

AJJewsbury said:

So these days a completely different approach is taken - rather than try to keep different "earths" apart, have just one and make it a low impedance as possible by having as many parallel paths as possible. Hence all the talk of meshes and rings and stars of meshes in BS 7671 and BS EN 50310. Some of the old language about separate "clean" earths does seem to cling on in some quarters though.

Yes absolutely, as you say trying to maintain a clean earth system is incredibly difficult, and a mesh achieves the same effect - we're just trying to stop any earth conducted noise from "dirty" devices turning into a significant noise p.d. between different equipment earths of sensitive equipment.

Sometimes however it's really difficult to make a thorough mesh, and there is a mix of very sensitive equipment and very noisy equipment and you do end up having to join the earths at just one point. As Jam says, it's not recommended "for general use" in 61000-5-2 (figure 4), but sometimes it is the only way. But that then becomes a real challenge to design and maintain - as 61000-5-2 also says "When properly installed and the topology maintained (their emphasis), this approach has been found satisfactory".

However as far as I'm aware (and from my experience), where "clean" earths are used it doesn't matter from the EMC point of view where the two earths are joined (and for safety they must be joined somewhere, as 61000-5-2 points out), as long as they are only joined at only one place.

I used to come across this with installing sound equipment in TV studios - massive 3 phase lighting dimmers and microphone amplifiers were not comfortable bedfellows! But like Andy says about IT equipment, it was not easy(!) ensuring that the installation was designed such that no-one could accidentally join the two earths at a second unintended place, which would have been electrically safe but potentially extremely noisy.

I am slightly surprised if IT systems these days still need a clean earth, but I'm no expert in the earthing or immunity of IT systems. I'm more used to them being the "dirty" systems that put 'orrible noises all over my nice clean (analogue) audio systems!

It's a really interesting point. And, as everyone has said, full of myths and rumours. But some of the rumours do have a grain of truth.

Cheers,

Andy

AJJewsbury said:

So these days a completely different approach is taken - rather than try to keep different "earths" apart, have just one and make it a low impedance as possible by having as many parallel paths as possible. Hence all the talk of meshes and rings and stars of meshes in BS 7671 and BS EN 50310. Some of the old language about separate "clean" earths does seem to cling on in some quarters though.

Yes absolutely, as you say trying to maintain a clean earth system is incredibly difficult, and a mesh achieves the same effect - we're just trying to stop any earth conducted noise from "dirty" devices turning into a significant noise p.d. between different equipment earths of sensitive equipment.

Sometimes however it's really difficult to make a thorough mesh, and there is a mix of very sensitive equipment and very noisy equipment and you do end up having to join the earths at just one point. As Jam says, it's not recommended "for general use" in 61000-5-2 (figure 4), but sometimes it is the only way. But that then becomes a real challenge to design and maintain - as 61000-5-2 also says "When properly installed and the topology maintained (their emphasis), this approach has been found satisfactory".

However as far as I'm aware (and from my experience), where "clean" earths are used it doesn't matter from the EMC point of view where the two earths are joined (and for safety they must be joined somewhere, as 61000-5-2 points out), as long as they are only joined at only one place.

I used to come across this with installing sound equipment in TV studios - massive 3 phase lighting dimmers and microphone amplifiers were not comfortable bedfellows! But like Andy says about IT equipment, it was not easy(!) ensuring that the installation was designed such that no-one could accidentally join the two earths at a second unintended place, which would have been electrically safe but potentially extremely noisy.

I am slightly surprised if IT systems these days still need a clean earth, but I'm no expert in the earthing or immunity of IT systems. I'm more used to them being the "dirty" systems that put 'orrible noises all over my nice clean (analogue) audio systems!

It's a really interesting point. And, as everyone has said, full of myths and rumours. But some of the rumours do have a grain of truth.

Cheers,

Andy