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Old 24th May 2020, 10:53 am   #39
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: How much DC on your mains ?

There are four main types of RCD in use (AC, A, F & D) with immunity to various undesirable forms of residual current. See: https://hager.com/uk/support/regulat...n-of-rcd-types

Anyone installing an RCD should consider the likely presence of leakage currents on the protected circuits other than the AC fundamental, and fit a device capable of both responding to such leakage and remaining correctly calibrated in its presence. Note that it is the DC component of the leakage that causes the problem, not that of the total load current, and therefore most likely to reach significant levels if a load leaks asymmetrically, rather than through linear leakage of a supply with a small DC component.

A historical note about egregious dumping of DC into the mains: One of the first attempts in the UK at a fully electronic theatre-lighting dimmer was Strand's 1949 thyratron-based unit that came to be known as 'Woody's Electronic' after J.T.Wood, its designer. Probably to sidestep a patent by Century in the USA who were working towards a similar goal, Wood started out by using three thyratrons per channel, one on each phase, to provide controlled 3-pulse rectification feeding variable voltage DC to the lamps. The entire current used by the installation was DC and the sum of the three line currents, totalling some hundreds of amps of DC, returned via the neutral! Not surprisingly, one of the first installations to be conmissioned (possibly Reykjavik Opera House) resulted in the neutral of the theatre's main supply cable burning out. It had other problems including thyratrons running away, so the company soon returned to electromechanical dimmers pending the arrival of thyristors.

Last edited by Lucien Nunes; 24th May 2020 at 10:58 am.
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