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Subject: Re: Earth v.s. circuit ground

Date: 08/15/00 at 6:06 PM
Posted by: Brian Richardson
E-mail: brian.m.richardson@baesystems.com
Message Posted:

In Reply to: Earth v.s. circuit ground posted by toysl on 08/07/00 at 4:23 AM:

The signal ground or chassis ground, whichever you are using is the local zero volts (or equipotential)plane and surge limiters limit transients with respect to this voltage.

The absolute earth voltage at particular point in a building may be different to that at another point due to different earth currents resulting from comon mode capacitors in line filters, phase imbalance. High frequency signals like transients on the earth line see a high impedance to ground due to the inductance of the wire.

One real danger arises when a piece of equipment faults to ground on the power circuit you're connected to causing the earth voltage to pulse to possibly hundreds of volts (we're talking microseconds here). The chassis of your equipment pulses high and capacitively couples the spike into the electronics of your equipment causing failure or data corruption. Your surge suppressors and common mode filters help save you from disaster by also pulsing your zero volts line in the electronics high to match, but the spike is now on the data line to the device at the other end and that's where their surge suppressors limit the spike with respect to their zero volts plane.

You have to look after your end and also limit transients in or out to a safe absolute limit, but as you can see, earths aint earths in this game.

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