A colleague asks “As you know, the leakage current test in the 950 based standards requires measuring leakage current in single phase equipment to line and neutral, rather than to a ground conductor. Is this done because earth and neutral are the same at the distribution panel and this is the way Europe did it, so the US/ Canada just followed along?”
The leakage current measuring circuits in IEC 950 and those in North America are EXACTLY THE SAME. The difference between measuring circuits is the point in the measuring circuit that is connected to ground.
Since only one point of the leakage current measuring circuit is connected to ground, there can be no current from the circuit to ground. Therefore, the grounded point of the leakage current measuring circuit plays no part in the leakage current measurement, is arbitrary, and even is unnecessary.
In the North American circuit, the grounded point is the supply side of the leakage current meter. This was chosen to make the measurement setup both simple and convenient.
In the IEC circuit, the grounded point is the EUT (Equipment Under Test) side of the leakage current meter. This was chosen to protect test personnel and to have one measuring circuit for measuring leakage current from all the various supply distribution schemes, TN, TT, and IT, and to be independent of whether the supply is polarized at the plug. (Refer to IEC 950, Sub-clause 1.2.12 for definitions of TN, TT, and IT.) Refer to IEC 950, Figures 13 and G1.
In North America, the leakage current meter is inserted in series with the protective grounding conductor. The neutral conductor remains connected to ground. Because the leakage current meter is a 1500-ohm resistance in the protective conductor, the EUT is not grounded during the test, and could be hazardous to the personnel in the test area. This is the situation of the ground being on the supply side of the leakage current meter. In IEC 950, the leakage current meter is connected in series with the neutral grounding connection. (It is not inserted in series with the protective grounding conductor.) The neutral conductor must be disconnected from ground. (This disconnection is facilitated by the use of an isolation transformer). The EUT remains grounded via the protective conductor. This means that the EUT is grounded during the test, and is not hazardous to personnel in the test area. This is the situation of the ground being on the EUT side of the leakage current meter.
In other words, IEC 950 inserts the leakage current meter between the neutral and ground. In North America, the leakage current meter is inserted between the EUT and ground.
IEC 950 did it this way because (1) some supply systems do not maintain polarity at the plug, (2) some supply systems use the IT system where the neutral is not directly grounded, (3) some supply systems use the TT system where the neutral and protective conductors have independent connections to ground (which would add resistance into the leakage current measurement), and finally and most important, (4) the work area remains safe during the test.
In either leakage current measuring circuit, IEC 950 or North American, you get EXACTLY THE SAME VALUE of leakage current. The other switch positions in IEC 950 (greater than 1 in the figures) provide the equivalent to the North American polarity reversal switch.
In other words, the IEC 950 scheme is a general scheme independent of the supply circuit, while the North American scheme is a simplified scheme suitable for use only on a TN supply with a polarized plug. This discussion raises the question of what is leakage current and where does it come from?
The use of the word leakage to describe this phenomenon is probably a misnomer. If we think of a leaky bucket, we tend to think of a bucket with small holes through which water leaks out. The word implies some sort of faulty situation.
The phenomenon we commonly call leakage current is NOT due to the equivalent of small holes in a bucket. Leakage current is NOT due to any sort of a fault. Leakage current is due to normal and predictable circuit parameters.
Leakage current arises from two physical phenomena: (1) insulation resistance, and (2) capacitance.
In constructing electrical and electronic equipment and products, it is quite common to use metal parts which are not part of the circuit. Over the years, UL has referred to these parts as dead metal parts. These dead metal parts are insulated from the active circuits by a combination of air and solid insulations.
Insulations do not have infinite resistance. Their resistance is very high and can usually be ignored, but they do have a finite value of resistance. This resistance can be measured with meters called insulation resistance meters.
Since insulations have resistance, they will conduct current in proportion to the source voltage and the value of the resistance. This current is the first source of leakage current.
The same dead metal parts separated from live parts by insulation, by definition, constitute a capacitor. Their capacitance cannot be ignored as such capacitance is distributed throughout the equipment and appears as a single capacitor. Since capacitors have reactance, they will conduct current in proportion to the source voltage and the value of the reactance. This current is the second source of leakage current.
In most situations, insulation resistance is so high that it can be ignored as a source of leakage current. Most leakage current is due to the distributed capacitance of mains circuits to dead metal parts. (In products with EMI filters, most leakage current is due to the real mains-to-ground capacitors of the EMI filter.)
Note that the source of leakage current is the mains voltage. If the voltage is known, and if the capacitance to dead metal parts is known, then leakage current can be predicted with reasonable accuracy. (It is interesting to contemplate on whether leakage current can or does arise from non-mains voltage sources in primary circuits or from secondary circuits.)
Or, putting it another way, the maximum value of capacitance can be calculated knowing the maximum allowable leakage current and the mains voltage.
If the leakage current limit is 0.5 milliampere, and the mains voltage is 120, 60 Hertz, then the capacitive reactance cannot be less than:
The capacitance cannot be more than:
On the other hand, if the leakage current limit is 3.5 milliamperes, and the mains voltage is 250, 50 Hertz, then the capacitive reactance cannot be less than:
The capacitance cannot be more than:
These values of capacitance are not likely in ordinary equipment and product construction. These values arise when there is a need for discrete capacitance between mains and dead metal parts as in an EMI filter
For the most part, the phenomenon known as leakage current approaches a current source. A current source is a source which provides a constant current regardless of load. Unfortunately, we have unduly complicated the measurement of leakage current by requiring a network across which we measure voltage and then calculate current. As a result, many errors are incurred in the measurement.
I advocate a simple measurement of the current in the grounding wire for determination of leakage current. Just put an ammeter in series with the ground wire. Unfortunately, this won’t give an accurate measurement when the current is from a voltage source rather than a current source. (A 1.5 volt battery will measure 1 milliampere leakage current using the traditional leakage current measuring schemes.) Some experts hypothesize that some of the leakage current in certain switching-mode power supplies is from a voltage source rather than a current source. Some leakage current measuring circuits have been devised to bypass current from a high-frequency voltage source. But, to my knowledge, no one has yet studied or published data on whether or not some leakage current is from a voltage source.
Copyright 1993 by Richard Nute Originally published in the Product Safety Newsletter, Vol. 7, No. 1, January-February, 1994
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.
