Hi Rick:
> I am working on a project that delivers 8 KV to 1" CRTs mounted on a
> helmet mounted display. The output of the high voltage supply is limited
> to 100uA and assuming that the supply complies with paragraph 2.4 (2.4.4
> in particular) Paragraph 2.4.1 states that:
Note that the output must not exceed 2 mA dc in both normal
condition and single-fault condition in the source.
You're okay for normal condition, but what is the output
with the worst-case single fault in the supply? I would
guess that the supply is quite simple, and any fault will
result in zero output.
>
> "Except as permitted in 2.4.6, segregation of parts of LIMITED
> CURRENT CIRCUITS from other circuits shall be as
> described in 2.3 for SELV Circuits"
>
> So what this tells me is if the stored charge is less than 45 uC
> (2.4.6), it is considered safe with only basic insulation? If over the
> stored charge limit, it must be treated as an SELV circuit and isolated
> accordingly.
No. The circuit is either a Limited Current Circuit or
it is not. If yes, then it can be accessible. If not,
then it must be treated as a hazardous voltage circuit.
There are two sets of insulation which you must consider:
1. The insulation between the energy source and the 8 kV
source. I would guess that the energy source is an
oscillator operating from SELV. I would guess that
the 8 kV is from a transformer and possibly a voltage
multiplier. So, the insulation between the energy
source (SELV) and the 8 kV is that of the transformer.
If the supply is SELV and the output is Limited Current,
then no safety insulation is required between the two
circuits. If the supply is not SELV, then the insulation
between the source and the 8 kV must be the same as for
the insulation between the source and SELV, except that
the voltage rating must be the sum of the 8 kV and the
supply source voltage.
2. The insulation between the body and the 8 kV. If the
circuit is a Limited Current Circuit, then it is
treated just as a SELV circuit, i.e., no insulation is
necessary between the body and the 8 kV. A Limited
Current Circuit can be accessible; it is the current-
limited image of the voltage-limited SELV.
> My application mandates that the anode wire be as small as possible, as
> it is bundled inside a larger cable grouping and is limited by the
> design of the helmet mount. In lieu of a standard HV anode wire I have
> found that a coax cable provides excellent performance when tested for
> dielectric strength. Upwards of 2 KV is possible without breakdown. I am
> considering grounding the shield to prevent static buildup and the
> possibility of an potential breakdown in the coax. So if this is in fact
> LIMITED CURRENT it seems to me the application is valid.
This is a different issue. You are asking whether or not an
insulation rated less than 8 kV can be used in this application.
For the purposes of safety, no insulation is required. However,
for reliability, it would be unwise to use an insulation rated
less than 8 kV as the insulation would be subject to early
failure.
Furthermore, despite the fact that the standard does not require
insulation for safety purposes, UL and most other certification
houses would require suitably rated insulation.
> My questions now are:
> 1. If this in fact LIMITED CURRENT, is basic insulation such as the coax
> adequate?
See above.
> 2. If the requirement is SELV, is it reasonable that the insulation
> between the center conductor and shield may meet the requirement for
> reinforced insulation?l
Yes and no. For safety purposes, a Limited Current Circuit is
equivalent to a SELV circuit; no insulation is required. A
Limited Current Circuit can be accessible. However, for
reliability -- especially with such high voltage -- the insulation
should be rated 8 kV minimum. (The long-term effects of partial
discharge will eventually destroy the insulation.)
> 3. To add additional safety should the coax shield be tied to chassis
> ground or should the shield be tied to the anode return?
If the coax is rated 8 kV or more center-to-shield, then the
shield can be tied to ground (anode return). If the coax is
not rated 8 kV, then don't use it (per above remark).
> I am especially concerned because this cable routes against the body
> between the helmet and supply, and is in close approximation to the
> head. Any thoughts or comments you may have would be appreciated.
I would guess that the head might be especially sensitive to
partial discharges that might occur in the air between the 8 kV
and the skin. I would certainly apply 8 kV insulation between
the body and the 8 kV. Even that may not be acceptable because
of the capacitive divider that can exist. The best solution is
a grounded barrier (e.g., coax) between the 8 kV and the body.
Best regards,
Rich
ps: Beware of x-radiation from this crt. Larger crt's operating
at 8 kV have relatively thick faceplates which attenuate the
x-radiation. Very small crt's have thin faceplates, and may
not attenuate the x-radiation to an acceptable level. Note,
too, that the 50 mm measurement distance rule would not apply
in this case.
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