For the purposes of safety, what is “working voltage,” and what is its relevance to the safety of the equipment?
SOME DEFINITIONS
Here are some definitions of “working voltage”:
IEC 950, First Edition, Sub-clause 1.2.9.6:
EN 60950, First Edition, Sub-clause 1.2.9.6:
IEC 950, Second Edition, Sub-clause 1.2.9.6:
UL 1950, First Edition, Sub-clause 1.2.9.6:
CSA 950, First Edition, Sub-clause 1.2.9.6:
“Working voltage: The highest voltage to which the insulation under consideration is, or can be, subjected when the equipment is operating at its rated voltage under conditions of normal use.”
28A(Central Office)29, Sub-clause 3.5:
(Revision of IEC 664, including IEC 664A)
“The highest RMS value of the AC or DC voltage which may occur (locally) across an insulation of equipment supplied at rated voltage, transients being disregarded, in open circuit conditions or under normal operating conditions.”
IEC 742, First Edition:
“Working voltage denotes the highest r.m.s. voltage which may occur across any insulation system at rated input volts, phase angle and transients being neglected, in no-load conditions or during normal operation.
“When considering the insulation system between windings not intended to be connected together, the working voltage is considered to be the highest voltage occurring on any of these windings.
“Attention is drawn to the fact that the working voltage to earth of the input may be different from the apparent value on single-phase systems with no neutral line and on three-phase systems with no earthed neutral when star connected, or when delta connection is used. The output voltage of a transformer may be artificially raised with respect to earth by conditions which occur in an appliance or equipment.”
IEC 335, Second Edition, Sub-clause 2.2:
“Working voltage denotes the maximum voltage to which the part under consideration can be subjected when the appliance is operating at its rated voltage and under normal conditions of use.
“Normal conditions of use include changes of voltage within the appliance imposed by likely occurrences such as the operation of a circuit breaker or the failure of a lamp.
“When deducing the working voltage, the effect of possible transient voltages on the supply mains is ignored.”
IEC 65,
IEC 348,
IEC 601-1,
IEC 1010.
“Working voltage” is not defined in these standards.
WORKING VOLTAGE
Except for IEC 335, we can conclude that working voltage is the voltage, exclusive of transient overvoltages, across an insulation under normal operating conditions.
From a safety point of view, the only insulations with which we are concerned are basic, supplementary, and reinforced. Therefore, working voltage (exclusive of transient overvoltages) is the voltage across basic, supplementary, or reinforced insulation under normal operating conditions.
Failure of a safety insulation could lead to an injury. Our objective is to prevent failure of basic, supplementary, or reinforced insulation due to normal operating conditions. Insulation is presumed to fail if the voltage applied to it exceeds its electric strength.
This discussion addresses how working voltage is used to predict the value of voltage applied to the insulation and how to determine that the insulation has electric strength greater than the applied voltage.
Thus, we can be assured that safety insulation will not fail as a result of the voltage applied to it.
ELECTRIC STRENGTH
Let’s first examine the relationship between working voltage and electric strength (hi-pot) voltage.
I like to think of the world as having two kinds of circuits.
The first kind of circuit is widely distributed and has many different loads, many of which are inductive or otherwise naturally generate and inject transient overvoltages into the circuit. Mains circuits are common examples of this kind of circuit. It is a normal condition that mains circuits have transient overvoltages.
Because transient overvoltages are normal in this kind of circuit, insulations used in the circuit must have an electric strength equal to or greater than the highest expected transient overvoltage for the circuit. One rule-of-thumb relating electric strength to mains voltage (i.e., working voltage) is the traditional 2V + 1000, where V is the rated (working) mains voltage.
For many years, 2V + 1000 was the standard formula for determining the electric strength voltage for insulations. Recent safety standards based on IEC 664 use tables to determine the electric strength voltage for any value of working voltage.
Note that, while transient overvoltages are normal conditions of mains circuits, the value of working voltage does not include such overvoltages. Instead, for mains circuits, the working voltage is the rated value of the mains voltage.
Using both the formula and IEC 664, we find that the required electric strength for mains working voltage of up to 250 volts is about 1500 volts.
The second kind of circuit is of limited distribution, has a limited number of highly controlled loads, and is suitably isolated from the mains circuits such that it has virtually no transient overvoltages. Equipment secondary dc circuits are examples of this kind of circuit. It is an abnormal condition that secondary dc circuits have transient overvoltages.
Insulations used in the second kind of circuit must have an electric strength equal to or greater than the highest working voltage for that circuit.
Consequently, an insulation with an electric strength greater than 1500 volts would be suitable for use
in a mains circuit with rated (working) voltage up to 250 volts.
That same insulation would also be suitable for use in a dc secondary circuit with a nominal (working) voltage up to 1500 volts.
Working voltage is the basis for determining the electric strength (hi-pot) voltage required of a basic, supplementary, or reinforced insulation.
SPACINGS
Now let’s examine the relationship between working voltage and spacings between conductors of the working voltage.
Electric strength is directly proportional to the distance through the insulating medium: the greater the distance, the greater the electric strength. The electric strength of most insulating media are rated in volts/distance. Therefore, spacings (distance through the insulating media) are an indirect measure of the electric strength of the insulating media.
(The volts/distance parameter also depends on the shape of the electric field in the insulating medium. The maximum volts/distance occurs with a “homogeneous” field, while the minimum volts/distance occurs with the worst-case “inhomogeneous” field.)
Commonly, safety standards publish tables working voltage and distances in air (clearances). So, working voltage is used to determine the clearance distance.
However, the clearance distance values in many of those tables are very much greater than the volts per distance value for air for the working voltage. Likewise, the clearance distance values are also greater than the volts per distance value for the transient overvoltage or hi-pot voltage.
So, while the working voltage is indeed used to determine the clearance distance from a table in the standard, often there is no physical or mathematical relationship between the clearance distance value and either the working voltage or the transient overvoltage or the hi-pot voltage.
Many safety standards have no requirements for distance through solid insulation. The hi-pot test is the only mechanism by which the solid insulation is evaluated and determined as not likely to fail. This is okay as almost any and every solid insulation of any usable thickness will have an electric strength greater than 3000 volts. According to one wag, even Mr. Whipple’s squeezingly soft Charmin has an electric strength greater than 3000 volts.
Some safety standards publish minimum values of distance for solid insulation, regardless of the volts per distance characteristics of the insulation. As with clearance distances, such values have no relationship to applied voltage. The interface between a solid insulating medium and atmospheric air insulating medium is a special case. Within the safety trade, this is commonly referred to as “creepage” distance. An example of this interface is the emergence of leads from the case of the optocoupler into the air.
This interface is of special concern because it is often subject to deposition of a third, uncontrolled material (i.e. pollution of the surface of the solid insulation). Therefore, the authors of safety standards have published various schemes by which the interface must have a greater dimension than is required for a pure air insulation. This greater dimension supposedly accounts for the lesser volts per distance value of the “foreign” material (as if we already knew the electric strength of the “foreign” material).
However, the failure of the interface (creepage) due to the accumulation of pollution is a long-term failure mechanism. Consequently, the electric strength of the interface (creepage) is based on working voltage rather than transient overvoltage. Therefore, when the working voltage is very much less than the transient overvoltage (as in a mains circuit application), the creepage distances requirements are much less than the clearance distances requirements. Similarly, when working voltage and transient overvoltages are equal, the creepage distances requirements are much greater than the clearance distances requirements.
Obviously, the greater of the two distances, creepage or clearance, takes precedence as the requirement for the interface (creepage) distance.
A construction of two conductors in air is also subject to pollution. In this case the pollution accumulates directly on the conductors, thus effectively reducing the distance in air (clearance) between the two conductors. This concern is principally directed at very small values of clearance (i.e., electric strength values less than 1500 volts rms) where the pollution could completely bridge the air gap.
Working voltage is the basis for determining creepage distances of a basic, supplementary, or reinforced insulation.
Transient overvoltage is the basis for determining the distance through air (clearance) and the distance through solid insulation for basic, supplementary, or reinforced insulation. Working voltage is the basis for determining electric strength sufficient to withstand normally-occurring transient overvoltages.
Recall that distance through insulation is an indirect measure of the electric strength of that insulation. Note that common safety standards independently specify hi-pot voltage (electric strength) and spacings (which determine electric strength). Any correspondence between the hi-pot voltage and the spacings is purely coincidental. Typically, safety standards require spacings such that the electric strength is very much greater than the hi-pot voltage.
However, IEC Publication 664 is an attempt to create correspondence between the hi-pot voltage and the distance through insulation.
CONCLUSION
From working voltage, we determine the value of the hi-pot test voltage, a direct measure of electric strength. From working voltage, we determine the minimum values of spacing, an indirect measure of electric strength. Therefore, working voltage is the basis for determining the minimum electric strength of insulations.
Copyright 1993 by Richard Nute Originally published in the Product Safety Newsletter, Vol. 6, No. 4, July-August-September, 1993
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.
