Fuse(d) Note

• To: tesla-at-pupman-dot-com
• Subject: Fuse(d) Note
• From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Tue, 29 Jun 1999 12:08:44 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi All,

> Original Poster: Tedd Payne <teddp-at-earthlink-dot-net>
> MA AC METER
> 0-50 mA AC, measure neon transformer current directly, metal
>enclosure with binding posts on top, fuse, good cond $15.00

 Snip

Just a thought for measuring freaks:

If you measure the short circuit current of a (limited) hv xformer
DO NOT use a fuse in the meter circuit!! While this may sound
silly at first (and might get some eyebrow raising), there is
reasoning behind this thought. A 50mA meter will be able to
handle around 150-200mA without being damaged. While the
needle will peg to the endstops, the coil and the mechanical
mechanism won´t be damaged by this. A 50mA fuse, on the
other hand (meter fuses are fast or SURE-BLO type), will
NOT be able to handle a 3-4x rating. If the meter fuse is your
typical 5x20mm (D) or 6x32mm (U.S.) kind, the open distance
will NOT stop the arc from a 12-15kV xformer. This WILL
fry your meter instantly, as the meter (coil) doesn´t have the
proper insulation to handle the hv (now) present.

The same goes for a DVM. ALWAYS measure the current
on the 10 or 20A range. This range usually isn´t fused,
(although some expensive units do have one) and the shunt
is of very low ohmage (0.01ohm or thereabouts). This will
act as a "full" short for a current limited xformer. The DVM
will take no damage because the voltage drops to very low
values (way under the rated 750-1000Vac of the meter).

Just to be safe, you shouldn´t touch the meter case. It IS
better to fry your meter (shouldn´t happen) than it is to fry
yourself. NEVER switch the DVM range while the xformer
is under power. This will ruin the DVM, as most DVMs use
a "break before make" type of rotary switch to change
ranges. This kind of switch will allow the voltage level to
rise up to very dangerous levels (during movement of the
contacts) that will overvolt your meter and destroy (it WILL
definately destroy the precision AC->DC rectifier) it. But
then, you shouldn´t be touching the meter case anyway....
............. ;o)


Coiler greets from germany,
Reinhard

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