5.3.4 Overload & Earth Leakage Toroids
Current transformers are not ideal devices and if correct procedures are not followed during installation,
nuisance tripping can result.
If, for example, we consider a single phase earth leakage system where active and neutral pass
through a toroid, then at all times currents in the two wires are equal and opposite so that the net
current through the toroid is zero. An ideal current transformer would have all the flux from each wire
contained in the core and so would accurately add the opposing fluxes to get a net result to zero. A real
current transformer has “leakage fluxes”. That is, a very small proportion of the total flux from each
cable is not contained in the core, but in the space outside it and as result it may link some turns but
not others, depending on the positioning of the cables.
The effect of this is that a small output may be obtained from the CT where none would arise if the
device were ideal.
The size of the error will vary from CT to CT of the same type because of slight differences in the core
and the symmetry of the winding.
Problems caused in this way become worse as CT sizes increase, as currents increase and a
decrease occurs in the symmetry of the cables. Nuisance tripping tends to occur when the total current
rises, such as when a large motor is started.
This is not normally a problem with the current levels found in flameproof enclosure applications. To
help avoid problems in other applications, select the smallest internal diameter CT, to suit the cable
size.
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