Operational Insulation in IEC 950

Normally, this column addresses technical issues in the field of safety. Usually, it does not address issues in safety standards. This issue’s topic is rather unique because this column addresses safety requirements for a non-safety critical device, “operational” or “functional” insulation.

IEC 950 specifies requirements for insulation in ALL circuits, including extra-low-voltage, and SELV circuits.

These requirements are of interest because the principal purpose of insulation is protection against electric shock.

(A secondary safety purpose of insulation is protection against short circuits which could lead to conductor or component overheating and fire.)

The insulations which are critical to protection against electric shock are defined as “basic,” “supplementary,” and “reinforced.” (In this article, I refer to these as safety insulations.) IEC 950 includes a fourth insulation, “operational insulation.”

“Operational insulation” is defined as the insulation needed for the correct operation of the equipment. A note accompanying the definition states that operational insulation by definition does not protect against electric shock. The note continues by saying that operational insulation may serve to minimize exposure to ignition and fire.

Examples of “operational insulation” would be all of the spacings and solid insulations in primary and secondary circuit that are not safety insulations, and all of the insulations within ELV and SELV circuits. IEC 950 specifies spacings (clearances and creepage distances) for all four insulations (basic, supplementary, reinforced, and operational) for both primary and secondary circuits.

Therefore, for IEC 950, you must consider every spacing and every solid insulation in every circuit throughout the entire product.

Some examples of operational insulation spacings (pollution degree 2) in primary and secondary circuits are shown in Table 1.

Table 1

Because of these spacings requirements, IEC 950 says that many printed wiring board insulations for the bulk dc in the primary of switching-mode power supplies must have at least 4.0 mm between conductors.

This is more than that required for primary-to-ground creepage distance, 2.5 millimeters!

IEC 950 says that SELV circuits on printed wiring boards must have at least 1.2 millimeters between conductors. If we were stuck with these dimensions, then we could never use SMT devices!

These spacings requirements provide many design constraints, especially as our products get smaller and smaller, and require smaller and smaller spacings.

Fortunately, IEC 950 provides some test and constructional alternatives to these dimensions.

The first alternative is to test the spacings with a dielectric strength test.

This is a good alternative because the required spacings are very much larger than the actual breakdown distances. Let’s look at some examples.

For 350-volt dc primary and secondary circuits, the test voltage would be 1500 volts rms.

For the ELV and SELV circuits, the test voltage would be 500 volts rms.

Upon review of IEC 664, we find that 1.1 mm will withstand 1500 volts, and 0.1 mm will withstand 500 volts. These data suggest that creepage distances very much smaller than the required creepage distances will pass the dielectric strength test.

So, all we need do is, for each voltage, find the smallest spacing on the board, and test it. For ELV and SELV circuits, if the spacings are not less than 0.1 mm, then we can expect to pass the 500-volt dielectric strength test. This will qualify all the ELV and SELV spacings on the board. Similar tests can be done for each voltage greater than ELV.

The second alternative is to short-circuit each insulation that is less than the required spacing.

This would be an inordinate amount of testing if each spacing had to be tested. Fortunately, IEC 950 specifies only two conditions when such testing must be performed. This cuts down the testing to a reasonable amount.

The first condition is where short circuiting would cause overheating of a material and thereby create a risk of fire. A short-circuit, by definition, is zero ohms and cannot itself be a risk of fire. The short-circuit testing specified in IEC 950 tests for heating in relatively low impedances in the source providing the current into the short-circuit. Usually, this will be the power supply or power distribution of the electronic equipment.

So, depending on resistances in the circuits, you may be able to conduct one short-circuit test which will maximize the heating in the power supply and power distribution circuits.

The second condition is where short-circuiting would cause thermal damage to one of the safety insulations, basic, supplementary, or reinforced. In low-voltage secondary circuits, these insulations would be the isolating insulations in the isolating transformer. So, a short-circuit at the output of the transformer or the power supply will cause the maximum heating of the primary-secondary (safety) insulations.

So, short-circuit testing is not as onerous as it first seems.

The third alternative is an exemption to both dielectric testing and short-circuit testing. If the material that could be overheated during the short-circuit test is V-l or better, then short-circuit and dielectric strength testing
is exempted.

Since circuit boards usually are V-1 or better, and wire insulation is V-1 or better, and since transformer insulations are usually V-1 or better, often testing is not required.

Furthermore, elsewhere in the IEC 950 standard, use of V-1 materials is encouraged for all insulations and wherever there is a chance of overheating.

So, after all this discussion, we discover that, for the most part, we can ignore spacing requirements for operational insulations, including most of those in primary circuits of switching-mode power supplies.

What do these operational insulation (spacing) requirements and alternative tests and constructions buy in terms of product safety?

Let’s first examine the purpose of spacings requirements. The spacings requirements in IEC 950 are loosely related to IEC 664. The object of IEC 664 is to prevent solid insulation failure under normal operating conditions. IEC 664 tells us that normal operating conditions include the transient overvoltages on power lines.

(The solid insulation in question is both bulk solid insulation and surface — creepage — insulation. Air insulation can be allowed to fail as air is a renewable insulation. The failure of air is not permanent and usually inconsequential to the safety of a product.)

IEC 664, being a basic safety standard, is principally concerned with dimensioning or otherwise protecting safety insulations against failure due to the normal transient overvoltages that may be transmitted via the power line
to a product.

When spacings requirements are applied to operational insulations, the presumption is made that (1) the solid operational insulation is indeed subject to transient overvoltage, and (2) the solid operational insulation will not fail in the presence of transient overvoltages. Prevention of failure of operational insulation presumes that the failure of operational insulation will lead to an unacceptable safety situation.

Since the safety insulations are independently addressed, and since safety insulations fully preclude electric shock, the unacceptable safety situation implied by the failure of operational insulation must be overheating and fire.

Now we can express why and how operational spacings, testing, and construction contributes to the safety of the product.

First, spacings and, alternatively, dielectric strength testing, demonstrate that the operational insulation is not likely to fail.

Second, short-circuit testing demonstrates that upon failure of operational insulation, unacceptable overheating and fire are not likely.

Third, the requirement for V-I insulating materials for operational insulation (without any testing) presumes a fire, but only of short duration. (The heat source ignites the V-1 material in proximity to the heat source; the V-1 material burns away from the heat source until the remaining
material is too far from the source to continue ignition, and the fire extinguishes.)

ACKNOWLEDGEMENTS

Thanks to a colleague for suggesting this topic.

Copyright 1995 by Richard Nute  Originally published in the Product Safety Newsletter, Vol. 8, No. 3, May-August 1995