Previous: EARTH LEAKAGE CIRCUIT BREAKER (ELCB)

EARTH LEAKAGE CURRENT METER/MONITOR

Typical Number in Hospital: 3

Cost Bands: 2

References:

Small electric currents leak from equipment driven from the mains electrical supply. These currents are mainly due to capacitance in the supply lead and mains transformer and are usually carried to earth via the protective earth conductor. There are agreed standards (e.g. IEC 601.1/BS 5724) for allowable currents which may flow in the protective earth lead or between other terminals on the equipment (e.g. patient connections and earth) during normal operation, and under single fault conditions.

An earth leakage current monitor is designed to measure these small currents either by insertion into the protective earth path or by connection between the terminals of the equipment and earth. IEC601.1/BS 5724 requires that the meter indicates combined a.c. and d.c. currents but that frequencies above 1 kHz are de-emphasized by a resistance/capacitance filter with a time constant of 6.7 ms. This is because higher frequencies cause less muscle spasm, and are thus less dangerous.

The allowable earth leakage currents depend on the class and type of the equipment as set out in Table 1.

TABLE 1 Allowable leakage currents

Equipment type

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Current path Type B Type BF Type CF

Earth leakage (mA) O.5 0.5 0.5

Enclosure to earth (mA) 0.1 0.1 0.01

Patient leakage (mA) 0.1 0.1 0.01

The allowable leakage currents from equipment with a single fault (e.g. broken earth lead) are as in Table 2.

Table 2 Single fault condition: allowable leakage currents

Equipment type

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Current path Type B Type BF Type CF

Earth leakage (mA) 1.0 1.0 1.0

Enclosure to earth (mA) 0.5 0.5 0.5

Patient leakage (mA) 0.5 0.5 0.05

These current levels are important if they flow inside the body and so clinical equipment is routinely checked for earth leakage and in some special apparatus these currents may be continuously monitored and alarm circuits operated if the levels exceed safe limits. For intermittent testing the supply earth connection is interrupted and an earth leakage current meter inserted into the lead. For continuous monitoring it is usual to detect the difference between the current flowing in the two supply leads, as in the earth leakage circuit breaker (ELCB) commonly used on bench supplies for testing electronic apparatus.