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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
wrote ... I should connect the resistor to the high voltage side and the meter to the low voltage side....please explain further....can't I just connect one lead from the meter to my cap bank's NEGATIVE terminal and the other meter lead to the resistor and the resistors remaining lead to my cap bank's positive terminal?
Those two statements mean the same thing, unless the positive terminal of your cap bank were grounded, in which case the resistor should go in the negative lead. The idea is to make sure the case and guts of the meter aren't connected directly to a high voltage terminal.
Having said that, if you're charging your cap bank off a NST, neither terminal can be grounded! Because the centre tap of the NST is grounded to the case, "ground" will end up being somewhere near the middle of your bank and the two ends of it will charge to 2.5kV above and below ground respectively.
Any attempt to ground either end will short out half of the NST and cut your final charging voltage in two (I think) and any attempt to lift the centre tap ground on the NST will make it flash over internally.
In a situation like this, I would put a 2.5MOhm resistor in each lead of the meter, and be careful not to touch it too! I have seen live meters mounted behind thick plexiglass in industrial gear.
Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Thanks Steve, hey the NST's I am using have only 2 terminals with no center tap, which is funny, it is like half of an NST if you know what I mean. There is the Positive mounted on the normal big porcerlain bushing and the other is grounded right to its case. So if the case is really ground (not really though right?, it seems its just made common potential by physical connection with one of the 2 terminals). If the case is really ground then the Positive would swing 2,000 V up and 2,000V down below with respect to the case as a center ground. But in reality if I ignore the case as ground and put a mter across the 2 terminals I'd get a 4,000 volt sine wave. SO I guess my choice is whether or not to ground the case to the equipment chasis or not. The meter will be mounted on wood and the NST's on wood, the base is wood there'll just be a sheet metal (lined with rubber I'm planning) chassis over it all but will touch nothing live. So I'll have to think about this pheww.
EDIT - I forgot to say, I like that suggestion of two 2.5 Megohm resistors one on each lead....if it's allrite I might do that anyways, seems safer. At least then if I DO touch the meter and it's leaky there's no way the cap bank can realease it's full fury of amps through me. Heck maybe I should invest in a newer plastic case meter...
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My new 1954 Kilovolt Meter
