Originally posted by: jencam
If one piece of equipment has no connection to mains earth and the casing becomes live then the fault current will flow through the cable shield into the other piece of equipment then down the earth wire connected to that same piece of equipment.
If the equipment has no mains earth, then it will be Class II or Class III as appropriate, and therefore the risk of the screening/shielding becoming live should
be very low (if not, the provisions of Class II and Class III haven't been met).
However, in a fault, casings of Class I equipment can (in theory) rise up to the full single-phase supply voltage with respect to the main earth terminal, and/or the "general mass of earth" outside.
In all pieces of equipment, earthed or not, there remains the risk of "impulses" appearing on communications cables, which needs to be addressed in specific installations.
The cable shield is not intended to convey large currents as it's only for EMI screening and it is likely to melt through before the mains fuse blows.
It depends on the type of screen: braid screens can carry relatively large currents, and it's likely PCB tracks (if used) will give way first.
In practice, though, what you find is that the current splits depending on the impedances of the conductors involved.
very basically, "Earth loops" are of three main types, of which a very swift overview is:
1. Signal path conductor is earthed at both ends, so you actually see the "noise" in the earth as part of your wanted signal. This can be reduced by keeping lenghts low, and/or providing additional bonding between the equipment (but there may be little benefit on longer runs, say longer than 3-10 m, if the noise causing a problem is at higher frequencies).
2. Screens carry lots of current which is then induced in signal cables. A good "common bonding network" and separation/segregation reduces the impact of this type of problem
3. Signal path conductor (or in some cases screens) are earthed at one end, and therefore act as a transmitter and/or receiver. (earthing at both ends, or providing capacitive earthing of the screen at the "isolated" end can help here - but if the screen is isolated for a safety reason, the capacitor needs to be correctly specified in terms of voltage rating, and safety performance, e.g. "Class Y").
However, external cabling have potentially the biggest risks. They are longer, therefore more susceptible to induced voltages, differences in ground potentials etc.
First, they have to withstand impulses from lightining strikes, HV switching and HV faults that are induced to signal cables. Even with relatively low capacitance, we need to consider that these phenomena contain high frequency components, so can still "jump" into the cables.
Secondly, these phenomena lead to "ground potential rises" - so the localised "earth" at one end of the cable is different to that at the other.
Does anybody know of any good sources of information about connecting up Class I equipment with shielded signal cables? My son is particularly interested in the arrangements used with professional video equipment and industrial automation.
Not sure there's a single point of information, to be honest, possibly because of the variety of factors involved, whether you are cabling "inside a building" or "between installations", etc.
It's sometimes seen as a "fact of life" that electrical/audio/telecomms/control engineers learn about this "on the job".
However, there's some good stuff in annexes to EN50174-3 on "external telecomms cabling", and
EN50310 is the current EU standard for earthing systems in buildings supporting this kind of equipment. However, it's an "ideal", aimed at reducing impedances in the building earthing system to keep circulating currents between equipment to a minimum (because impedances of screens etc. will be far more than the impedance back through the protective & supplementary earthing systems).
There are a number of products in the market place for audio and CCTV etc. systems that use common ground, to overcome the "earth loop", and information from the suppliers, along with information from surge protection suppliers, is often a good read.
EUR ING Graham Kenyon CEng MIET TechIOSH
Principal and Proprietor,
G Kenyon Technology