'Clean Earths' Aren't Clean! Or Earth!
01 Oct 2020
'Clean Earths' Aren't Clean! Or Earth!
All conductors have self-inductance and space-charge capacitance, which causes them to have self-impedances that vary with frequency. Apart from superconductors, all conductors also have bulk-resistance, but above a few hundred Hz this is generally swamped by their self-inductance.
For example, a 20 mm diameter copper rod 300 mm long has a series impedance of 16 µΩ at 0 Hz. However, at 1 kHz its series impedance is 1,280 µΩ, and at 1 MHz it is 1.28 Ω.
All conductors have mutual inductance and mutual capacitance to other conductors. This causes them to couple noise voltages and currents into, and from, those other conductors. We often call this near-field / close-field coupling, or crosstalk.
The voltages and currents in all conductors interact with their local electromagnetic fields, whether we want them to or not. I.e. all conductors behave as transmitting and receiving radio antennas.
For example, a 20 mm diameter copper rod 300 mm long is a perfect electric field transmitting and/or receiving antenna at around 250 MHz (monopole mode) or 500 MHz (dipole mode).
So even if Earths, Grounds, Chassis, etc., could provide ‘waste disposal’ sink-holes for unwanted currents (which they can’t, see below), no conductor that connects to them could be free from noise. Even if they are wide tin-plated copper braid straps, or huge busbars.
Even if their insulation is coloured green, or green with yellow stripes!
No conductors of finite size can ever be called ‘clean’.
People mistakenly assume that a large conductive structure called Earth, Ground, Chassis, Frame, Hull, Fuselage, Deck, etc., is an infinite sink for currents at any frequency.
But this is impossible because all currents, including stray, parasitic, sneak, leaked, noise, differential-mode, and common-mode currents, always flow in closed loops. Always have, always will.
There can be no ‘waste disposal’ sink-holes, for any currents.
Huge amounts of time and cost are wasted ever year by electronic designers mistakenly assuming that large structures called Earth, Ground, Chassis, Frame, Hull, Fuselage, Deck, etc., are important for either the functionality or the EMC of their circuits.
Electronic circuits need conductors for voltage references, and for carrying loop return currents (whether they are wanted, or stray ‘noise’ currents). These conductors might, or might not be connected to Earth, Ground, etc., for safety, power distribution or other non-circuit design reasons.
Of course, connecting a part of any electronic circuit to a large conductive structure has an effect that must be taken into account for good EMC design. But the large structure – whatever it is called – is not necessary for good EMC design.
We should only use words like Earth, Ground, Chassis, Frame, Hull, Fuselage, Deck, etc. – and their relevant symbols – to mean the actual points of electrical connection to those large conductive structures, and not for circuit design.
MORE DISCUSSION 1: Earth Loops, Ground Loops, and Hum Loops
But what about the traditional use of single-point connections to so-called ‘Clean Earths/Grounds’, to deal with Earth Loops, Ground Loops, and Hum Loops?
I discussed this in blog about ground loops so won’t repeat it here.
MORE DISCUSSION 2: So-called clean earths/grounds encourage damage
A long (say > 30m) separated conductor in a building connected to the same rods buried in the soil as the building’s safety earth/ground structure, can easily develop a transient voltage difference along its length of 10kV or more with respect to that safety earthing/grounding structure.
The transient voltage could induced by lightning that happens within a kilometre or so, or it could be induced by the flyback or field-collapse caused by disconnecting a large inductive load.
When different parts of an electronic circuit are connected to both a safety earth/ground and to a (so-called) clean earth/ground, unless the circuit’s insulation can cope with 10s of kilovolts (which it usually can’t), these transients and surges can damage the electronics.
Some people connect their so-called clean earth/ground wires to a separate rod from the ones used by the safety earth/ground structure. Now, nearby lightning-to-ground strikes can cause the transient voltage difference between the ‘clean’ and the safety earth/ground structures to exceed 100 kV.
Almost no electronic equipment is made with insulation that can withstand such high voltages, and so damaged electronics is even more likely.
The result is that the use of so-called clean earths/grounds is a very common cause of damaged electronics all over the world. It is a very significant annual financial loss. Transient protection devices can be added to try to prevent this from happening, but why bother when, these days, clean earths/grounds provide no real benefits anyway?
(Duplicated, multiple, meshed safety earth/ground conductors generally create safer structures, too.)
All because some designers believe that mythical ‘clean earths/grounds’ are real.
They might as well believe in rainbow unicorns!
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