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alanblaby:
Yes, I'd like to know the theory too.
We installed a large DB in a hospital suite to supply a single scanner, and its associated auxilaries.
The DB/Panel was custom made by T. Clarke. It was fed direct from the substation by a 5 core SWA cable, iirc, 185mm, though it may have been 95mm, its 2 years ago now.
Inside the panel was the usual earth bar and outgoing circuit connections on it, then a separate 'Clean Earth Bar' which had a number of 10mm cables going to almost all machines in the room.
Being as the earth was supplied direct from the substation to the DB, then how can that 'clean earth' be any different to the normal earth bar?
Multiple connections (what used to sometimes be called "cross-bonds") reduce inductance. This is what's shown in Fig A444.2, Fig A444.3 and A444.4 in BS 7671 - and is also recommended by BS EN 50310.
Chris Pearson:
I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.
Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.
gkenyon:Chris Pearson:
I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.
Graham, thank you. A bit over my head!
Chris Pearson:gkenyon:Chris Pearson:
I am curious to know why these additional earthing systems use such large cables and earth bars - much bigger than a CPC.Less inductance - the frequencies we are talking about are far above 50 Hz and because the effect of inductance is proportional to frequency, this is the driver.
Graham, thank you. A bit over my head!
Well, I can explain it another way.
The impedance of an inductor Z is given by the formula
Z=2πfL where
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