You are presumably assuming that, unlike me, one doesn't have a TT system! I fear that my kettle would end up closer to 230V than 115V for that 30ms whilst one was praying that the RCD did what it said on the tin (with TT, I doubt that the fault current would be high enough for the MCB to trip).The voltage which will appear briefly in your kettle example will depend upon the resistance of the conductive paths involved. If the resistance between the live side of the supply and the point of fault contact is equal to the resistance from the metal casing back to the earthed side of the supply, for example, then the voltage which appears on the kettle's casing will be half the supply voltage, or about 115V.
Prior to the faulty circuit being disconnected, there will be a touch voltage of approximately half the supply voltage between the kettle and a metal tap. The purpose of main and supplementary equipotential bonding conductors is to reduce the magnitude of this voltage during the period of the earth fault. See http://electrical.theiet.org/wiring-matters/22/bonding.cfm?type=pdf[/QUOTE]
Lest anyone gets confused, it's perhaps worth pointing out that this link relates to BS7671:2001, so some of what it says, particularly about supplementary bonding, is a little obsolete.
Kind Regards, John
so if the resistance is unequal it could be higher or lower value of voltage ?
also how does the touch voltage 50 v max work if i touched the kettle and say a metel tap at time of fault ?
thanks for the anawers
You don't need to worry about the 50v rule where disconnection occurs quickly, within 0.4s for the given scenario.
could anyone explain the potential divder earth part, which means half supply voltage is shown at the fault ?
in either case of 230 or 115 volts at the fault is that saying i would recieve a shock at one of the two voltages but if lots of fault currunt was to flow i would be ok as the shock duration is so short ie 0.4 s ?
JohnW2 said:You are presumably assuming that, unlike me, one doesn't have a TT system! I fear that my kettle would end up closer to 230V than 115V for that 30ms whilst one was praying that the RCD did what it said on the tin (with TT, I doubt that the fault current would be high enough for the MCB to trip).
could anyone explain the potential divder earth part, which means half supply voltage is shown at the fault ?
I would have thought the full 230v flows (earth fault loop path) and as long as the circuit Zs if below BS 7671 tabulated values for that circuit, the protective devices should open and clear the fault.
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