Chapter A General rules of electrical installation design

F - Protection against electric shock
8 Residual current devices (RCDs)
Recommendations concerning the installation of RCDs with
separate toroidal current transformers
The detector of residual current is a closed magnetic circuit (usually circular) of
very high magnetic permeability, on which is wound a coil of wire, the ensemble
constituting a toroidal (or ring-type) current transformer.
Because of its high permeability, any small deviation from perfect symmetry of
the conductors encompassed by the core, and the proximity of ferrous material
(steel enclosure, chassis members, etc.) can affect the balance of magnetic forces
sufficiently, at times of large load currents (motor-starting current, transformer
energizing current surge, etc.) to cause unwanted tripping of the RCD.
Unless particular measures are taken, the ratio of operating current Ι∆n to maximum
phase current Ιph (max.) is generally less than 1/1,000.
This limit can be increased substantially (i.e. the response can be desensitized) by
adopting the measures shown in Figure F73 , and summarized in Figure F74.
Schneider Electric - Electrical installation guide 2008
L = twice the diameter of
the magnetic ring core
Fig. F73 : Three measures to reduce the ratio Ι∆n/Ιph (max.)
Measures Diameter Sensitivity
(mm) diminution factor
Careful centralizing of cables through the ring core 3
Oversizing of the ring core ø 50 → ø 100 2
ø 80 → ø 200 2
ø 120 → ø 300 6
Use of a steel or soft-iron shielding sleeve ø 50 4
b Of wall thickness 0.5 mm ø 80 3
b Of length 2 x inside diameter of ring core ø 120 3
b Completely surrounding the conductors and ø 200 2
overlapping the circular core equally at both ends
These measures can be combined. By carefully centralizing the cables in a ring core
of 200 mm diameter, where a 50 mm core would be large enough, and using a sleeve,
the ratio 1/1,000 could become 1/30,000.
Fig. F74 : Means of reducing the ratio IΔn/Iph (max.)
L
F41
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