Measuring a grounding system means connecting leads to the grid and to remote probes. If a fault occurs or lightning strikes during the test, thousands of volts can appear between them. IEEE 81 lists the risks: voltage on the electrode, leads running parallel to energized lines, surge arresters and disconnected neutrals.
Short answer. A grounding system is measured with leads that run from the grid to distant probes. If a ground fault occurs in the substation at that moment, or lightning strikes, a potential difference that can reach thousands of volts appears between the grid and the remote points. IEEE Std 81-2012 devotes its clause 5 to these risks. This article explains what they are and why they exist; it does not replace your company’s safety procedure or that of the facility operator.
Why there is voltage on the electrode under test
When a ground fault injects current through the grid, the grid potential rises with respect to remote earth. That rise is the ground potential rise, or GPR. Clause 5.1 indicates that it can be on the order of several thousand volts, and that the step and touch voltages around the electrode, the test equipment and the remote grounds can also be lethal.
The risk does not depend only on faults. The leads to the current and potential probes reach points that represent remote earth. The guide asks that they be treated as if voltage could exist between them and any point of the substation grid. System faults and lightning are the main concern. In addition, the signal the equipment injects into the remote current electrode can produce significant touch voltages.
What risks the guide lists
| Situation | Risk | What IEEE 81 states |
|---|---|---|
| Electrode and test leads connected to the grid | Lethal voltage between the electrode and remote earth; potential transfer during a fault | Clause 5.1 |
| Remote probes out of sight or in a public area | People touch a live lead or probe | Clause 5.1 d) |
| Test leads parallel to a line carrying high current | Induced voltage in the lead, with risk of severe shock to anyone in series with one end and a probe | Clauses 5.1 e) and 7.4.1 |
| Surge arrester ground continuity test | The base can approach line potential; lightning or switching discharges above 50,000 A; system fault if the arrester fails during the test | Clause 5.2 |
| Disconnecting neutrals and shield wires | Hazardous voltages from current in the interconnected wires or coupling with other circuits, with the line energized or not | Clause 5.3 |
| Disconnecting the neutral of energized equipment | High voltages that can injure personnel or damage equipment | Clause 5.4 |
| Test equipment with output voltage above 50 V | Shock if someone touches an energized conductor; the public must be protected | Clause 6.2 |
Most test instruments deliver 50 V or less. If an instrument with a higher voltage is used, the guide itself asks for extra care with the potentials on electrodes and leads.
The precautions the standard describes
Clause 5.1 lists general measures. A test plan is drawn up and reviewed with the personnel taking part. Hands and other body parts must not close the circuit between points of possible high potential difference. Dielectric gloves and footwear reduce the risk when handling leads that come out of the grid. Exposed leads and electrodes are insulated from workers and the public before the signal is applied and before they are connected to the grid. The signal is applied for short periods and the leads are removed as soon as the test ends. The guide clarifies that other precautions may be needed.
For leads that run parallel to an energized line, the guide indicates that the physical orientation of the leads, grounding, or both can mitigate the induced voltage.
Observer with a radio
Remote probes can be out of sight of the test personnel, or their leads can cross an area accessible to the public. In that case they are monitored continuously by an observer in radio contact with whoever operates the equipment. The guide asks for this while a test signal is applied or remote potentials exceed 50 V. It also warns that one or more leads connected to the grid can transfer potentials well above 50 V during a fault, so the observer is needed as long as the leads remain connected to it.
What this article covers and what it does not
This guide explains the real risks of the measurement, which are the same in any energized installation. It does not say how to set up a test, which protections to use or which personal protective equipment values apply. The standard itself warns that its documents are not intended to ensure safety: whoever applies a guide is responsible for determining and following the applicable safety practices, laws and regulations. In an energized substation, those practices include the operator’s work permits and the company’s procedures.
If you need to understand why a grid can raise its potential, see the guides on step and touch voltage and on the substation grounding grid. If the test includes surge arresters, see the guide on surge arrester leakage current and the one on lightning shielding. To understand how shock occurs, see Electric Shock in a Substation and Tolerable Body Current per IEEE 80. Potentials that arrive through cables and pipes are explained in Transferred Voltage: Cables, Pipes and Fences, and readings that are falsified during the test in Earth Measurement Errors.
We measure your grounding system
At Tiza Group we measure grounding systems in energized substations and plants, in Colombia, or elsewhere on request, with a test and safety plan agreed with the facility operator. See the testing and commissioning service or write to us through contact. In the quotation we recommend the test and the equipment your case needs.
Related guides
- Earth Measurement Errors: Stray Currents and Coupling
- Ground Potential Rise (GPR): What It Is and How to Calculate It
- Transferred Voltage: Cables, Pipes and Fences
- Electric Shock in a Substation: How and Why It Happens
- Step and Touch Voltage: Difference and IEEE 80
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: clauses 5.1 to 5.4 (safety precautions), 6.2 (test electrodes and test voltage) and 7.4.1 (interference and warning about leads parallel to high currents).
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.
Frequently asked questions
Can there be dangerous voltage on the grid if there is no fault?
Yes. IEEE 81 warns that a lethal voltage can exist between the electrode under test and remote earth under routine conditions, and also if a system fault occurs during the test. In addition, the test signal injected into the remote electrode can itself produce significant touch voltages.
Why is an observer with a radio needed during the test?
The remote probes and their leads can be out of sight or in an area accessible to the public. The guide asks for continuous monitoring by an observer in radio contact with whoever operates the equipment, whenever a test signal is applied or remote potentials exceed 50 V.
Can the ground of a surge arrester be disconnected to measure it?
Not while its primary terminal remains connected to an energized line. The base of the arrester can take a potential close to that of the line. The guide states that the ground of a surge arrester is never disconnected under that condition, for any reason.