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:
"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.
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.
My questions now are:
1. If this in fact LIMITED CURRENT, is basic insulation such as the coax
adequate?
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
3. To add additional safety should the coax shield be tied to chassis
ground or should the shield be tied to the anode return?
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.
Rick Busche
Evans & Sutherland
[email protected]
