Transferred Voltage: Cables, Pipes and Fences

Short answer

Transferred voltage is the voltage that a metallic conductor carries to a remote point, or from it, during a ground fault. It can equal or exceed the total ground potential rise of the grid and, since designing the grid for that contact is impossible, IEEE 80 asks to avoid it by isolating, separating and labeling the elements that leave the substation.

Short answer. During a ground fault, a metallic conductor leaving the substation, such as a pipe, a cable screen, a fence, a rail or the neutral of a low-voltage circuit, can carry the grid potential to a place where the soil is at a different potential. Whoever touches that conductor at the remote point receives a potential difference that can equal or exceed the substation GPR. This is called transferred voltage, and IEEE Std 80-2013 considers it a serious hazard.

What it is and why it is special

The standard defines transferred voltage as a special case of touch voltage in which a voltage is transferred to or from a remote point outside the substation site. It occurs, for example, when a person inside touches a grounded conductor at a distant place, or a person far away touches a conductor connected to the grid. What happens to a person who touches an element at a potential different from that of the ground they stand on is explained in electric shock from a ground fault in a substation.

Under those conditions, the potential difference can equal or exceed the full GPR of the grid, instead of the fraction found in a normal touch. With voltages induced in unshielded communication circuits, shield wires or pipes, it can even exceed the sum of the potential rises of two substations. That is why clause 8.2 concludes that it is not practical, and often impossible, to design the grid to limit the contact caused by those external voltages.

Where the potential leaves

Transfer path	How it occurs	What IEEE 80 proposes
Communication circuits	Cables and terminal equipment that end up at high voltage with respect to the local ground	Isolation and separation; optical fiber and optical isolators to separate the substation terminal from the remote one
Rails	Incoming rails carry the GPR to a remote point, or bring remote potential inside	Remove the rail sections at the perimeter; use insulating joints as support, not as the main measure
Low-voltage neutrals	The neutral of feeders serving external points is bonded to the substation ground	Isolate the neutral from the grid in the substation and treat it as a live conductor, or extend the grid to the remote service point
Portable equipment	Equipment and tools supplied from the substation and used outside the controlled-potential zone	Isolation transformer, local ground at the work site or portable generators
Pipes	Metallic pipes and ducts connected to the grid	Insulating sections long enough that the adjacent soil does not bridge them
Auxiliary buildings	Buildings tied to the substation by pipes, cable screens or telephone lines	Treat them as part of the substation, or separate them with suitable transformers and neutrals grounded only at the building
Fences	Fences extended to other areas of the site and connected to the grid	Isolate the fence from the substation it leaves, with long insulating sections so nobody can bridge them

Rails, pipes and fences

For rails, the guide recommends removing the perimeter sections after their initial use, or installing removable sections. Insulating joints are used to give the greatest possible protection while the rail is in use, but they are not the main measure: they can create dangerous situations. If the rail beyond the joint is in contact with the soil, a dangerous voltage can appear between that section and a rail or fence bonded to the grid.

For metallic pipes and ducts, the guide asks to always connect them to the grid to reduce hazards inside the substation. The transferred potential is reduced or stopped at the boundary if insulating sections of sufficient length are inserted, able to withstand the potential difference between remote earth and the substation.

For fences that extend beyond the substation, the guide recommends isolating the section of the substation from the one that leaves it, with insulating sections long enough that nobody can bridge them. A potential profile shows that, moving away from the area of influence of the grid, the touch voltage on the fence is no longer safe.

Low-voltage neutrals and service circuits

If a low-voltage feeder serving points outside the substation has its neutral bonded to the substation ground, all or a large part of the potential rise can appear at the remote point between that neutral and the soil. To avoid it, the neutral can be isolated from the ground at the substation, provided this does not lengthen the clearing of low-voltage faults so much that the total hazard increases. This is done with an isolation transformer. That neutral must then be treated as a live conductor: insulated to withstand the GPR and located away from people’s reach. The alternative is to treat the remote point as an extension of the grid, with a ring and rods or an equipotential grid that controls the voltages there.

Control cable screens

The metallic screens of cables can reach dangerous voltages with respect to ground if they are not properly grounded, because of insulation faults, electrostatic induction, screen currents or the potential rise during faults. Unless otherwise recommended to eliminate circulating currents in low-voltage or control circuits, the screens of shielded control cables are grounded at both ends, to reduce induced potentials. If the connection points are far apart, the large gradients in the grid during a fault can make excessive currents flow through the screen; one solution is to lay in parallel a bare copper conductor connected to both points. Unshielded cables suffer transient induced voltages of 190 % or more of those of shielded ones. For optical fiber aspects in communications, see single-mode and multimode fiber in substations.

Separate grounds

The practice of having separate grounds within a substation is little used, for reasons that clause 17.8 lists: the resistances are higher than with a single system, in an insulation fault high currents could flow through the safety ground, the coupling between nearby electrodes prevents keeping the GPR of the safety ground low, dangerous potentials can appear between close points and large transient differences occur between components during lightning or other overvoltages.

We carry out your grounding study

We review which conductors, pipes, fences and circuits leave your substation or plant, calculate the potentials they can carry outside and define the isolation or separation needed. See the engineering design and studies service or write to us through contact.

References

IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 3 (definition of transferred voltage), 8.2 (typical shock situations), 17.5 (grounding of control cable screens), 17.8 (separate grounds) and 17.9 (transferred potentials, subclauses 17.9.1 to 17.9.7).
IEEE Std 487, cited in IEEE Std 80-2013, clause 17.9.1, for details on the protection of communication circuits.

Frequently asked questions

Why can transferred voltage be greater than a normal touch voltage?

Because the person is between a conductor that brings the grid potential and a soil that is at a potential close to zero. Instead of the fraction of the GPR found in a normal touch inside the substation, they receive the full potential or more, if there are induced voltages.

Can the grid be designed to limit transferred voltage?

Not practically. IEEE 80 states that it is impractical, and often impossible, to design the grid for the contact caused by external transferred voltages. The risk is controlled with isolation, with neutralizing devices and by treating those circuits or pipes as if they were energized lines.

Is it a good idea to have separate grounds for safety and for the system?

The guide says they are rarely used for several reasons: higher resistances result, high currents could flow through the safety ground in an insulation fault, there is coupling between nearby electrodes, dangerous potentials can appear between close points and large transient differences occur during lightning.