Resistor Value for Measuring Leakage Current

Furor and controversy are words which describe the process by which standards committees decide the value of the resistor in the leakage current measuring network.

However, the different specified resistor values create no more than a 6.25% error for the value of the leakage current.

More furor and controversy surround the selection of the resistor tolerance. The resistor tolerance creates almost the same percentage error in the measured value.

Still more furor and controversy occur when we compare the ANSI, UL, CSA and IEC measuring circuits.

The ANSI, UL, CSA and IEC circuits are demonstrably identical; all four give the same measured value.

Resistor Value

Different standards specify different values for the current-sampling resistor in the current-measuring circuit for electric shock current and leakage current. Examples of these different values are shown in Table 1.

Table 1

What difference do these values make?

Let us assume that we are measuring 0.5 milliampere of leakage current from a 120-volt product. To have leakage current we must have a circuit consisting of a voltage source, a series impedance, the current-sampling resistor (1500 ohms), and a return path (ground). See Figure 1.

Figure 1: Leakage Current Circuit

We know E (120 volts) and I (0.5 mA). Using Ohm’s law, the total resistance in the circuit, including the 1500-ohm current-sampling resistor is:

R = 240,000 ohms

Subtracting the 1500-ohm current-sampling resistor, we have a source resistance of 238.5 kilohms. Using this value, we can calculate the current when using other values of current-sampling resistor.

And, we can repeat the calculations for a 240-volt source.

And, we can repeat the calculations for 3.5 milliamperes and 5.0 milliamperes leakage current.

What do these data mean? Essentially, we have a current source. This means that the current is nearly independent of the load which, in this case, is the current-sampling resistor.

The worst-case error is +6.25%. This means that a manufacturer could test leakage current with an ordinary ammeter, knowing that the ammeter reading is higher than the reading with a 1500-ohm resistor. If a manufacturer used the ammeter and the actual limit value, 0.5, 3.5 or 5.0 milliamperes, he would have a small guard-band such that his measurements would always be pessimistic.

So, where only power-line frequency appears in the leakage current, why go to the trouble of using the resistor? If it passes with the ammeter, it will pass with the resistor!

Why all the fuss about the value of the resistor?

Resistor Tolerance

Let us assume that we are again measuring 0.5 milliampere of leakage current from a 120-volt product. Recall from the discussion of resistor value, the source impedance is 238.5 kilohms when leakage current is 0.5 milliampere and the current-sampling resistor is exactly 1500 ohms.

In this case, assume the current- sampling resistor is a 1500-ohm, 5% resistor. Let us further assume that the resistor is at the low end of its tolerance, -5%. The resistor value therefore is 1425 ohms. Using Ohm’s law, the current in the circuit is:

I = 0.5002 milliampere

The actual voltage across the 1425-ohm resistor is:

E = (I) (R)
E = (0.5002) (1425)
E = 0.713 volts

If we now calculate the value of leakage current using the nominal value of the resistor rather than the actual value, we get:

I = 0.475 milliampere

This is very nearly the same error as the resistor tolerance, 5%.

Measuring Circuits

The UL and IEC measuring circuits are shown in Figure 2a. In a progression of figures, the circuits are simplified to their essential elements-ultimately showing the equality of the UL and IEC circuits.

Figure 2a: Original Circuits, UL – IEC

Figure 2b adds the source to the UL circuit as is already shown in the IEC circuit. Note that the UL circuit has its neutral grounded, while the IEC does not. The IEC circuit has the equipment grounded, while the UL does not.

Figure 2b: Add Source to UL

Figure 2c deletes the ground from both the UL and the IEC circuits. Since there is only one connection to ground in both circuits, there can be no current in the ground, so the grounding is extraneous to the measurement.

Figure 2c: Delete Grounding

Figure 2d simplifies the UL circuit by deleting the plug and socket.

Figure 2d: Simply UL

Figures 2e and 2f show the normal and reverse polarity positions, respectively, of the UL and IEC polarity switches.

Figure 2e: Normal Polarity

Figure 2f: Reverse Polarity

Capacitor

Next, let’s examine the effect of the 0.15 microfarad capacitor in parallel with the current-sampling resistor. Capacitive reactance is given by:

X_C = 17.7 kilohms

The parallel network of 17.7 kilohms and 1.5 kilohms resolve to an impedance of 1.38 kilohms. This is less than 10% effect at 60 hertz.

The capacitor is useful only when the leakage current includes high-frequency currents, which the capacitor serves to shunt around the current-sampling resistor. If the capacitor is not used, then the measurement is higher than it would be with the capacitor.

Conclusion

The value of the current-sampling resistor in measuring leakage current at power-line frequencies is of negligible consequence to the measurement. The use of an ordinary ammeter will always give a pessimistic and worst-case value for leakage current. If your product has an acceptable leakage current with an ammeter, then it will have an acceptable leakage current with the standard current-sampling measurement circuit. And, there is no difference between the UL and IEC measuring circuits. Perhaps furor and controversy are not necessary after all!

Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations. Mr. Nute holds a B.S. in Physical Science from California State Polytechnic University in San Luis Obispo, California. He studied in the MBA curriculum at University of Oregon. He is a former Certified Fire and Explosions Investigator.Mr. Nute is a Life Senior Member of the IEEE, a charter member of the Product Safety Engineering Society (PSES), and a Director of the IEEE PSES Board of Directors. He was technical program chairman of the first 5 PSES annual Symposia and has been a technical presenter at every Symposium. Mr. Nute’s goal as an IEEE PSES Director is to change the product safety environment from being standards-driven to being engineering-driven; to enable the engineering community to design and manufacture a safe product without having to use a product safety standard; to establish safety engineering as a required course within the electrical engineering curricula.