1. GENERAL
Generally, leakage current clamp testers are used to supervise the insulated conditions
in AC low voltage circuit. Those leakage clamp testers can measure the leakage current
of grounding line without cutting off the electricity of low voltage circuit on live power
condition.
Nowadays, however, many harmonics and high-frequency currents are contained to
leakage current of grounding line due to an increase in high-frequency electric
appliances with inverter, etc. and the measured values by different instruments are not
same even at the same measuring point.
Also, leakage current contains the resistive component (Ior) which flows to the ground
by insulation resistance and the capacitive component (Ioc) which flows by electrostatic
capacity. The actual insulation deterioration is caused by Ior but normal leakage clamp
testers cannot judge what kind of leakage current is flowing.
Under the circumstances, there are two kinds of instruments :
(1)
Igr measuring apparatus with low frequency injection method.
(2)
Ior measuring apparatus with simplified effective method.
In the low frequency injection method, a certain frequency different from 50/60Hz is
piled up by the injection transformer and then, the in-phase current is extracted from
the following current. This apparatus is inconvenient to carry and is very expensive.
On the other hand, in the simplified effective method, Ior is measured by extracting the
in-phase component from leakage current based on the voltage phase. This method
cannot detect the insulation condition of grounding phase and it is getting difficult to
take the voltage phase in recent transformation facilities and in the circuit of 3 phase/
3 wires, it is not reliable due to the error caused from unbalance capacitance to the
earth.
Our newly developed Ior clamp tester does not need the voltage input and can measure
Ior values of the grounding line as well as the lines of feeder simply by clamping CT.
Model MCL-400IR is the first and epoch-making leakage clamp tester.
