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On Limited-Current Circuits

A Product Safety Newsletter reader asked about where limited-current circuits are required by IEC 950, Sub-clause 2.4.1. He noted that a 5-volt DC circuit will give 2.5 milliamperes into the 2 kohm test resistor. 2.5 milliamperes DC exceeds the allowed 2.0 milliampere limit. The PSN editor then asked when it is helpful to use a limited-current circuit rather than a SELV circuit.

Let’s see if we can answer these questions.

Unfortunately, IEC 950 is not quite clear on the nature of a limited-current circuit and its role in preventing electric shock. Furthermore, it appears that various certification houses likewise are not clear on the nature of limited-current circuits and their role in preventing electric shock. On the other hand, IEC 1010 presents the idea of a “limited-current” circuit in a different light: In Sub clause 6.3.1.1, IEC 1010 specifies that if the voltage exceeds 30 volts rms AND the current exceeds 0.5 milliampere rms, then the circuit is considered hazardous.

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Note that we must have two, simultaneous conditions: first, the voltage must be greater than 30 volts rms, and second, the current available from that voltage must be greater than 0.5 milliampere (for IEC 1010). If both of these conditions are met, then the circuit is considered hazardous. However, if only one of the conditions is met, the circuit is considered not hazardous.  Let’s look at this in graphical form.

1510_TS_fig1

In most cases, where the voltage exceeds 30 volts rms, we PRESUME the available current exceeds the 0.5 milliampere criterion. For example, we do not measure the available current from

a mains circuit because we KNOW that the available current exceeds 0.5 milliampere. Normally, we simply identify all voltages exceeding 30 volts rms as hazardous. We don’t look at the other dimension, current.

Note the region below 30 volts. There is no current limit. In this region of the graph, any voltage source less than 30 volts is non-hazardous, including a 100-ampere car battery. (I am ignoring the concept of energy hazard, which has been discussed before in this column.) Now note the region below 0.5 milliampere. There is no voltage limit in this region of the graph, the circuit is said to be current-limited.

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A current-limited circuit is one where the open-circuit voltage exceeds 30 volts, but the available current (through an appropriate resistor) does not exceed either 0.5 milliampere or 3.5 milliampere, depending on the standard. For example, if we connect one end of a 240 kilohm resistor to 120 volts, the other end now constitutes a limited-current circuit. The maximum available current is:

I = 120 volts 240 kilohms

I = 0.5 milliampere

But, we need not have a resistor to develop a current-limited circuit. In some cases, insulation provides the current-limiting.

Consider the measurement of leakage current. If you open the ground and measure the voltage with a high-impedance voltmeter, you will measure about one-half the mains voltage (about 60 volts on a 120-volt system). (Try it!) So, we have 60 volts rms, which exceeds our 30-volt limit. We next measure the available cur- rent, and determine that it is less than 0.5 milliampere (or 3.5 milliampere, depending on the standard). What we have done is to show that the open ground is a current-limited circuit.

Another example of a current-limited circuit is the voltage source for some electroluminescence panels. The voltage may be in the neighborhood of 300 volts dc, but the current, when measured with an appropriate resistor, is less than 2 milliamps (depending on the appropriate standard). If you were to touch such a circuit, you would not likely have any sensation of current or electric shock.

Another “use” of the limited-current circuit is the “floating” circuit. A floating circuit is one where neither pole of the circuit is connected to ground.

Consider the case of a 5-volt circuit which generates the power for a floating power supply whose output is 150 volts. In this case the 150 volts is not current-limited pole-to-pole. However, because it is “floating”, it is current-limited from pole to ground. Thus, one need only provide basic insulation on each pole of the power supply to provide adequate protection against electric shock, including that of failure of one basic insulation.

A brief note about SELV, Safety Extra-Low Voltage, and how it differs from ELV, Extra-Low Voltage, and its relation to Limited-Current circuits. A 9-volt battery is ELV. It is considered safe by virtue of the value of the voltage being low.  A 9-volt battery-eliminator plugged into a wall outlet is also ELV. The 9-volt output voltage is considered safe by virtue of the value of the voltage being low (i.e., less than 42.4 volts dc). However, for a 9-volt battery eliminator, the voltage is derived from a hazardous voltage source. The low voltage must be isolated from the hazardous voltage. Therefore, the output voltage must be SELV.

The difference between ELV and SELV is that SELV is derived from a hazardous voltage source AND is suitably isolated from that source.

An ELV source can also be derived from a hazardous voltage source BUT need not be suitably isolated from that source. The 9-volt battery cannot be SELV because it has no hazardous voltage source from which it need be isolated.

The point is, that, for Extra-Low Voltage, safety is provided by the low voltage itself. Indeed, because the voltage is low and therefore non-hazardous, we can consider the conductors to be accessible parts which just happen not to be grounded (as is the case for most other conductive parts). However, where the ELV is derived from a hazardous voltage source, and where that ELV may constitute accessible conductive parts, the circuit must be suitably isolated from the hazardous voltage just as any other accessible conductive part must be isolated from the hazardous voltage. Usually, we simply install double or reinforced insulation between the ELV and the hazardous voltage, and verify its adequacy with measurement of spacings and testing for hi-pot.

This insulation between the ELV and the hazardous voltage is analogous to the insulation between the ground and the mains (hazardous voltage) circuits. Consequently, we can measure leakage current from the ELV conductors just as we would from the grounded conductors: open the ELV ground and insert the leakage current meter. This measurement shows that the insulation between the ELV and the hazardous voltage is a LIMITED-CURRENT CIRCUIT with respect to the hazardous voltage.

Therefore, an SELV is an ELV with a Limited-Current Circuit between it and its hazardous voltage source. SELV has TWO voltage sources. One is the low voltage itself. The other is the hazardous voltage from which the low-voltage is derived. With respect to the low-voltage source, the magnitude of the voltage renders the circuit conductors safe. With respect to the hazardous voltage, the insulation between the ELV and the hazardous voltage renders the circuit conductors safe. The insulation between the ELV and the hazardous voltage is an insulation between two conductors. Two conductors separated by insulation constitute a capacitor. The capacitor makes the path between the ELV and the hazardous voltage a limited-current circuit. Neither spacings measurement nor hi-pot testing evaluates the current through the capacitance.

Therefore, to complete the evaluation of the adequacy of the separation, we should test the SELV circuit for limited current from the hazardous voltage. We don’t normally do this because, usually, the capacitance is very low and can be neglected. Nevertheless, a product can be built which passes the spacings and hi-pot criteria, but does not pass the limited current criterion.

The concept of the limited-current circuit is extremely valuable as it is a necessary piece of protection from electric shock. In Figure 1, we usually only think of the voltage axis when we think of electric shock. Add to your understanding of electric shock by thinking also of the current axis and limited-current circuits.

Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.

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