Circuit
Centre to centre; the cable overall diameter for touching trefoil.
Mean of the screen inner and outer diameters.
Length between bonding points, not the whole route.
Optional. Balanced fault case only.
65 V is the usual GB figure (ER C55). Confirm the limit with the network operator.
Standing Voltage Against Section Length
The voltage rises linearly with section length, so where the curve crosses the limit is the longest single-point bonded section the circuit allows. Beyond it the screen must be cross bonded, sectionalised, or fitted with sheath voltage limiters. The fault figure is the balanced three-phase case only; earth faults change the problem and are not covered here.
Calculation Method
Trefoil, and the centre phase in flat formation
E = ω · 2×10-7 · I · ln(S / rm)
With balanced currents summing to zero, the contributions of the two other phases collapse into a single logarithm of the ratio of spacing to mean screen radius. The centre cable of a flat group is equidistant from both outers, so it gives the same answer as trefoil at the same spacing.
Outer phases in flat formation
E = ω · 2×10-7 · I · √(L² + L·ln2 + ln²2), L = ln(S / rm)
An outer cable is one spacing from the centre phase and two from the far phase, so the cancellation is incomplete. The result is roughly 20 % above the centre phase, which is why the outer phases set the section length.
What this does and does not include
- · Balanced, positive-sequence load current at the stated frequency. No harmonics.
- · Equal spacing in flat formation, and a single circuit. Parallel circuits induce into each other and raise the answer.
- · The screen open-circuit, i.e. single-point bonded. A solidly bonded screen carries circulating current instead, which lowers the voltage but adds loss and derates the cable.
- · No transposition. Cross bonded systems need the residual of three minor sections, which is a different calculation.
- · Earth faults, sheath voltage limiter energy duty and induced voltage into parallel metalwork are outside the scope of a free tool. They need a bonding study.
Background: CIGRE TB 283 and TB 347 on the earthing and bonding of single-core cable systems, and ENA ER C55 for the GB touch-voltage limit.
Frequently Asked Questions
What is induced sheath voltage?
When single-core cables carry load current, the alternating magnetic field links the metallic screen of each cable and induces a longitudinal voltage along it. Where the screen is earthed at one end only, that voltage appears as a standing voltage at the open end and rises in proportion to the length of the bonded section.
What is the limit for standing voltage on a cable screen?
In GB distribution practice the figure applied to a single-point bonded screen under normal load is commonly 65 V, per ENA Engineering Recommendation C55. Some utilities and clients specify lower, such as 50 V, and higher values are sometimes permitted where the screen is not accessible. Confirm the limit with the network operator before fixing section lengths.
Why is the outer phase worse than the centre phase in flat formation?
The centre cable sits at equal distance from both outer cables, so their contributions partly cancel and the result matches trefoil at the same spacing. An outer cable is one spacing from the centre phase but two spacings from the far phase, so the cancellation is incomplete and the induced voltage is roughly 20 % higher. The outer phases govern the section length.
What do I do if the standing voltage is too high?
Shorten the bonded section, tighten the spacing, or move from single-point bonding to cross bonding so the three minor sections cancel. Sheath voltage limiters protect the outer sheath at the open end but do not reduce the standing voltage itself.
What spacing should I enter for touching trefoil?
The cable overall diameter. Three touching circles of equal diameter have centres forming an equilateral triangle whose sides equal that diameter.