Grounding of Switch Handles, Arresters and Service Areas

Short answer

Besides the grid, the yard of a substation has points with their own exposure: the service areas inside the fence, the disconnect switch handles, where the operator stands, and the arresters. IEEE 80 asks for short, direct connections on arresters, a ground mat or a local grid at the handle, checking the voltages in service areas, and not using separate grounds inside the substation.

Short answer

A well-designed grid keeps step and touch voltages within safe limits over the whole area it covers. But there are points in the yard that deserve separate review, because people are next to them or because their ground connection conditions the protection of the equipment. IEEE Std 80-2013 treats them in its clause 17: service areas, disconnect switch shafts and handles and arresters, and also recommends not using separate grounds inside the substation. The general design procedure is in substation grounding design with IEEE 80.

Service areas

The step and touch voltages to which people are exposed outside the fence are very similar to those inside. Sometimes a fence is installed that encloses an area much larger than the one the substation uses, and the grid is built only in the used part and along the fence. The rest, unprotected, is often used as storage, a switching yard or a general service area. There the step and touch voltages must be checked to decide whether more grounds are needed.

A reduced grid, which does not include the service area, costs less at the start and avoids working around an already installed grid when the substation is expanded. In exchange, it protects people less, and because it has less area and less conductor, it has a higher resistance than a complete grid that includes the area.

The service area can also be enclosed with a separate fence, not bonded to the grid; the transferred voltage problems this can bring are treated by the standard in its clause 17.3. Elements that leave the grid can carry potential outside it, as explained in transferred voltage and transferred potentials.

Disconnect switch handle and shaft

Disconnect switch handles are a major concern when they are not properly grounded. Manual operation requires the operator to be next to a grounded structure, and several things can produce a fault on that structure and expose the operator to shock:

opening an energized circuit,
a mechanical failure,
electrical breakdown of a switch insulator,
trying to interrupt a line charging or transformer magnetizing current larger than the switch can safely interrupt.

Protecting against those risks is relatively easy when the handle is inside an extensive grid designed per the standard, because the step and touch voltages near it should be within safe limits. Even so, it is common to take additional measures to give the operator more safety. For example, connecting the switch shaft to a ground mat on which the operator stands. The mat is bonded directly to the grid and to the shaft, which gives a direct bypass to ground in parallel with the person operating. The path from the shaft to the grid must have the cross-section to carry the fault current for the required time.

How other utilities do it

A worldwide survey from 2009 cited by the standard showed that 82 % of the responding utilities required the shafts of air disconnect switches to be grounded to the grid, and that all took extra precautions to reduce surface gradients where the operator stands. The way of grounding the shaft was split almost equally: about half used a direct jumper between the shaft and the mat, and the other half a jumper from the shaft to the neighboring steel structure, using the steel as part of the path. About 90 % used a braid, and the rest a device without a braid. To reduce gradients under the operator, a grounded platform, a tight wire mesh under the surface material or a smaller spacing in the main grid was used.

Arresters

Arresters, or surge arresters, must always have a reliable, low-impedance ground connection. The standard states:

Connect them as close as possible to the terminals of the equipment they protect.
Keep the phase and neutral conductors as short and straight as possible.
On transformers, breakers and regulators, connecting the phase arrester to the tank minimizes the overvoltage on the equipment insulation with respect to ground.
The arrester neutral conductor must have the shortest and most direct possible path to the grid, to dissipate the surge energy.
Bends in those conductors increase the impedance noticeably and reduce the protective level.

Some utilities have separate ground conductors for arresters mounted on metal structures; others use the mounting structure or the tank of the protected equipment as the ground path, because the large cross-section of the steel offers less resistance than a copper cable of usual size. In that case a good electrical connection must be ensured between the structure and the arrester conductor and the grid, and it must be verified that the steel cross-section has the necessary conductivity and that there are no high resistances at the joints from paint, rust or other causes.

How the arrester works is in surge arresters; its protection against direct lightning, in lightning shielding.

Why separate grounds are not used

The standard says that the practice of having separate grounds inside the substation is hardly used, for five reasons:

The resistances are higher than with a single ground system.
In insulation faults in the substation, high currents can flow through the safety ground.
With high coupling between separate electrodes in the same area, the ground potential rise of the safety grounds cannot be kept low in line faults.
Hazardous potentials can appear between nearby grounded points, because the systems can decouple to a certain degree.
Large transient potential differences can occur between equipment components during lightning or other surges, with incorrect operation or damage.

How we do it at Tiza Group

Reviewing the voltages in service areas, the disconnect switch handles and the arrester grounds is done with the grid model and the fault data. At Tiza Group we include it in the design of medium, high and extra-high voltage substations. Learn about our engineering design and studies service or write to us. We deliver it remotely, worldwide.

References

IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 17.1, 17.2, 17.7 and 17.8.

Frequently asked questions

Why is the operating shaft of a manually operated disconnect switch grounded?

Because the operator stands next to a grounded structure, and a fault on that structure would expose the operator to shock. The causes can be opening an energized circuit, a mechanical failure, electrical breakdown of an insulator, or trying to interrupt a line charging or magnetizing current larger than the switch can safely interrupt.

How short must the ground conductor of an arrester be?

As short and straight as possible, both on the phase connections and on the neutral. Bends in those conductors can add significant impedance and reduce the arrester's protective level.

Can a separate safety ground be kept apart from the system ground inside the substation?

The standard says it is rarely used. Separate grounds give higher resistances, do not prevent high currents from flowing through the safety ground in an insulation fault, do not keep the ground potential rise of the safety grounds low, and can produce dangerous potential differences and large transient differences between components during lightning and other surges.