Hello from San Diego:
Today's question is, "What are the criteria for an insulator?"
There are two answers. One is from the physics or electronics
point of view. The other is from the safety point of view.
1. Physics and electronics.
No material is a perfect insulator. Similarly, at any
temperature above absolute zero, no conductor is a perfect
conductor.
All insulators have a finite value of resistance, albeit
very, very high.
Insulation resistance meters are especially designed to
measure the resistance of insulation. They're the same as
an ohmmeter except they use a much higher source voltage.
Philosophically and practically, the value of the insulation
resistance needed in a particular circuit depends on the
circuit characteristics. If you are dealing with a low-
impedance circuit, then insulation resistances of 10 megohms
and higher are acceptable. If you are dealing with megohm
circuit impedances, the you will need insulation resistances
of 10 gigaohms and higher.
2. Safety.
There are at least three parameters to consider when
considering safety, insulation resistance, certification,
and dielectric withstand.
Some of the older safety standards specify basic insulation
as having an insulation resistance of at least 7 megohms,
and supplementary or reinforced insulation having an
insulation resistance of at least 10 megohms. Not particularly
high.
You can perform the insulation resistance test with a meter
having a source of at least 500 volts dc.
In the USA and Canada, our certification houses require virtually
all safety insulations (i.e., basic, supplementary, reinforced)
be certified by one or more certification houses. The set of
criteria varies from application to application. One common
criterion is temperature. Most insulation criteria are valid
up to the rated temperature of the material.
You also need to perform a dielectric withstand test, where the
test voltage is specified by the applicable component or product
standard.
If you attempt to perform a dielectric withstand test on ferrite
core material, you will find that ferrite is a good conductor!
So, for safety purposes, ferrite must be treated as a conductor.
Best regards,
Rich
ps: Likewise, if you perform a high-current test to prove that
ferrite is grounded for the purposes of safety, you will
find that ferrite is a poor conductor!
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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