KMP_78250C_D03 Page 2 of 5

78250 Series

MAX250/MAX251 Compatible Converter Transformers

www.murata.com

TECHNICAL NOTES

ISOLATION

VOLTAGE

‘Hi Pot Test’, ‘Flash Tested’, ‘Withstand Voltage’, ‘Proof Voltage’, ‘Dielectric Withstand

Voltage’ & ‘Isolation Test Voltage’ are all terms that relate to the same thing, a test

voltage, applied for a specified time, across a component designed to provide electrical

isolation, to verify the integrity of that isolation.

All products in this series are 100% production tested at their stated isolation voltage.

A question commonly asked is, “What is the continuous voltage that can be applied

across the part in normal operation?”

For a part holding no specific agency approvals both input and output should normally

be maintained within SELV limits i.e. less than 42.4V peak, or 60VDC. The isolation test

voltage represents a measure of immunity to transient voltages and the part should

never be used as an element of a safety isolation system. The part could be expected

to function correctly with several hundred volts offset applied continuously across the

isolation barrier; but then the circuitry on both sides of the barrier must be regarded

as operating at an unsafe voltage and further isolation/insulation systems must form

a barrier between these circuits and any user-accessible circuitry according to safety

standard requirements.

REPEATED HIGH-VOLTAGE ISOLATION TESTING

It is well known that repeated high-voltage isolation testing of a barrier component can

actually degrade isolation capability, to a lesser or greater degree depending on materi-

als, construction and environment. This series has toroidal isolation transformers, with

no additional insulation between primary and secondary windings of enamelled wire.

While parts can be expected to withstand several times the stated test voltage, the iso-

lation capability does depend on the wire insulation. Any material, including this enamel

(typically polyurethane) is susceptible to eventual chemical degradation when subject to

very high applied voltages thus implying that the number of tests should be strictly lim-

ited. We therefore strongly advise against repeated high voltage isolation testing, but if

it is absolutely required, that the voltage be reduced by 20% from specified test voltage.

This consideration equally applies to agency recognised parts rated for better than

functional isolation where the wire enamel insulation is always supplemented by a

further insulation system of physical spacing or barriers.