Ampcontrol Pty Ltd
– ABN 28 000 915 542
ELV-PRO USER MANUAL
MAG-219 Version 5
– NOVEMBER/2020
Uncontrolled Copy - Refer to Ampcontrol Website for Latest Version
Page 48 of 59
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APPENDIX B: ELV-PRO CURRENT TRANSFORMERS
B1 Earth Leakage Toroids
Toroids (current transformers) are not ideal devices and if correct procedures are not followed during
installation nuisance tripping can result. Consider, for example, 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 toroid would have all of the flux from
each wire contained in the core and so would accurately add the opposing fluxes to get a net result of
zero. A real toroid 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 a 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 toroid where none would arise if the device were ideal.
The size of the error may vary from toroids of the same type because of slight differences in the core and
the symmetry of the winding. Problems caused in this way increase as the toroid size increases, as
currents increase and symmetry decreases. Nuisance tripping tends to occur when the total current
rises, such as when a large motor is started. The following guidelines would help to avoid such
problems.
B2 Toroid selection
1. Select the smallest internal diameter toroid, which will allow the cables to fit through. Avoid very
large toroids (>200mm aperture) or toroids with square apertures.
2. Only use approved toroids specified by Ampcontrol as these have been designed to minimise
problems.
B3 Toroid installation guidelines
1. Keep cables as close to the center of the toroid as possible. Do not tie them to one side of the
toroid. Remember to aim for symmetry.
2. Do not bring the cables back past the toroid within one diameter of the toroid. Trying to cram
cables into a small space reduces symmetry and may lead to problems.
3. Avoid placing the toroid near any device that produces magnetic fields. This includes bus bars,
transformers or other cables. Try to maintain several toroid diameters clearance.
4. Many small cables tend to be worse than say, three large ones. Try to position the toroid in the
circuit with this in mind.
5. Toroids used for core balance earth leakage protection cannot have bus bars passed through
them.
6. To prevent possible nuisance tripping it is suggested that the conductor screen of the earth
leakage toroid should be earthed at one end only, the relay end. If both ends are earthed the
possibility exists for the shield to become an earth loop, having finite resistance and injecting
noise into the toroid leads
