Robisan1 wrote:
>
> I have a copy of MIL-P-55110A, dated 18DEC64 which
> has the requirement that insulation resistance be
> 500 meg. so that value has been around for 34 years.
> As to exactly why that number was derived, I don't
> know for sure. I do know that it was based on a
> spacing of 26 mils.
>
> There are other requirements for various spaces for
> other specifications - such as Bellcore. And they do
> make sense.
>
> To use other sizes of comb patterns, especially closer ones,
> we probably need to change that value.
>
> Perhaps some of the design guys might have a clue as
> to the origin of the number.
>
> One thing to keep in mind is that on SIR testing for the
> assembly documents, there is a visual assessment after
> and there can be no corrosion or arcing etc to successfully
> pass the test, NO MATTER WHAT RESISTANCE VALUES
> YOU OBTAIN.
>
> Susan Mansilla
> Robisan Lab
>
> ################################################################
Susan, and other technetters
The 500 Mohm was based on best avialable technology many, many decades
ago. At the time 500 Volts was a maximum reasonable test voltage that
available power sources could reliably produce without having dielectric
strength/breakdown problem. Coupled with that,at the time, 10-25
microampere D'Arsonval analog meter movements were about the best
available technology, this means the minimum ampere per division was ~1
microampere; guess what, using Ohms Law it's equal to 500 Mohm.
IMO, it's been carried on (especially through military documents) for
all these years because it works, and it has a lot of good history,
lesser values of insulation resistance were more subject to failure, or
"increased risk for malfunction".
Now that applications voltages are being reduced to typically <5 Vdc,
perhaps the insulation resistance requirements for these low voltge
application should be re-evaluated by electrical/electronic engineering
personnel. But!!!! some applications use high-impedance (resistance)
input circuitry and it can be very susceptable to insulation resistance,
and don't forget insulation resistance is more than just surface, it
includes bulk/volume resistance of the base material.
Ralph
--
Ralph Hersey
Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN/FAX: 925.454.9805
e-mail: [log in to unmask]
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