When Should Earthing Be Measured? OHS & Legal Obligations

Earthing is the most fundamental life-safety measure in an electrical installation. During a fault it safely diverts leakage current to ground, protecting both people and equipment. That's why regularly measuring and reporting the earthing system is a legal obligation — and a more critical audit item than most facilities realize.

Why Does Earthing Matter This Much?

When an insulation fault occurs — say, a motor winding touching its casing — the earthing system routes that leakage current to ground through a low-resistance path. This lets the circuit breaker or residual-current device trip quickly, and prevents anyone touching the casing from being exposed to a dangerous voltage. If earthing resistance is too high, fault current doesn't flow fast enough, protection devices don't trip in time, and the casing can remain at a dangerous voltage.

Why is it Mandatory?

Occupational health & safety regulations require the electrical installation and earthing system at workplaces to be checked periodically. Skipping the measurement:

• Increases the employer's liability in a work accident
• Creates a direct non-conformity in audits
• Can lead to an insurance claim being refused
• Raises fire risk — weak earthing sets the stage for arcing in faulty circuits

How Often?

Earthing resistance isn't a fixed value; it changes over time with soil moisture, corrosion and mechanical effects. Resistance rises in dry seasons, electrodes corrode over time, and structural changes (excavation, new installations) can disturb the earthing network. So earthing measurement should be repeated periodically — typically annually or at the interval the regulation prescribes. In new installations it's always measured before commissioning; it's a standard part of the provisional-acceptance file.

How is it Measured?

• Earthing resistance measurement: typically via the three- or four-electrode fall-of-potential method, measuring the true ground resistance of the earthing electrode.
• Continuity measurement: we verify electrical continuity between panels, equipment casings and the earthing bar — a broken connection goes unnoticed until it's measured.
• Thermal inspection: where relevant, a thermal camera checks connection points for heating, an indicator of a loose connection or high resistance.
• Loop impedance: circuit loop impedance is measured to confirm that residual-current devices and fuses will actually trip in time under a real fault condition.

What SOREAS Earthing Measurement Covers

• Earthing resistance and continuity measurements
• Panel and equipment protective-earthing checks
• Additional thermal/electrical measurements where needed
• Turning the results into a compliant official report
• Concrete improvement recommendations where there are non-conformities (adding electrodes, renewing connections, extending the network)

What Happens if the Result Doesn't Conform?

If the measured resistance exceeds the limit set for the facility type, it's recorded as a non-conformity in the report along with an improvement recommendation. In most cases, the fix is driving an additional earthing electrode, strengthening the bonding between electrodes, or extending the earthing network. After the fix, the measurement is repeated to verify the result and the report is updated.

Who Issues the Report?

The earthing measurement report must be issued by an authorized electrical engineer — a measurement taken with an uncalibrated device, or without an engineer's signature, isn't accepted as valid in an audit. SOREAS performs the measurement with EMO-registered engineers and provides a valid, defensible report for audits.

How Earthing Resistance Limits Vary by System Type

Acceptable earthing resistance differs conceptually depending on the system type the facility is fed under:

• TN-S: the neutral and protective earth (PE) conductors are kept separate throughout the entire installation. Fault current returns via a low-impedance metallic path, so the earthing resistance needed for protection devices to trip reliably can generally be assessed with more tolerance — what matters most is the continuity of the PE conductor.
• TN-C-S: the neutral and PE are combined as a single conductor (PEN) for part of the installation and split after the incomer. In this system, the integrity of the PEN conductor and the accuracy of that split point matter just as much as earthing resistance itself; the measurement also checks that the split point is correctly implemented.
• TT: the facility has its own earthing electrode, independent of the utility's earthing. Protection here relies heavily on the sensitivity of the residual-current device (RCD) together with the facility's own earthing resistance — which is why, in TT systems, the earthing resistance value is a parameter that must be assessed jointly with the protection device's trip threshold, and it has a direct bearing on life safety.

Which system applies, and the acceptance criteria that follow from it, are defined in the facility's connection agreement and existing project documentation; verifying this before measurement is the first step to interpreting the result correctly.

What Happens During a Site Visit

On an earthing measurement visit, a SOREAS engineer follows these steps:

1. Preliminary review: the facility's earthing system type, prior measurement reports and any project drawings are reviewed.
2. Visual inspection: the physical condition of electrodes, connection points and earthing bars (corrosion, looseness, damage) is checked on site.
3. Disconnecting electrodes (if needed): an accurate fall-of-potential measurement may require temporarily isolating the electrode being measured from the rest of the system.
4. Taking measurements: earthing resistance, continuity and, where needed, loop impedance are measured with calibrated devices, and site conditions (soil moisture, season) are noted.
5. Evaluating results: measured values are compared against the acceptance criteria for the facility's system type and protection devices.
6. Reporting: results, along with any non-conformities and improvement recommendations, are turned into an official report and delivered with the engineer's signature.

Routine Measurement vs. Post-Incident (Forensic) Measurement

Periodic routine measurement follows a pre-planned schedule, aimed at tracking the earthing system's condition over time and documenting regulatory compliance. A measurement taken after an accident or fault serves a different purpose: the question isn't "is the facility compliant overall?" but "did the earthing system behave as expected at the moment of the incident, or was the earthing system itself a contributing cause?" This kind of measurement is carried out with the incident scene disturbed as little as possible, examining every connection point and protection device relevant to the event separately. Because the results often serve as evidence for an accident investigation, insurance claim or legal proceeding, the measurement and reporting are documented in far more detail, and the conditions at the time of the incident — which equipment was running, weather conditions, prior maintenance records — are included in the report. As a result, a forensic-grade measurement takes longer than a routine one and requires a much more thorough site examination.

FAQ

How often should earthing be measured? The general practice is an annual periodic measurement, though the facility's risk class and specific regulatory intervals can shorten that. Newly built or structurally modified facilities need an additional measurement.

Does production need to stop for the measurement? Usually not. Most of the measurement can be done without a power outage; some continuity checks need only a brief, planned interruption.

What should earthing resistance be? The acceptable limit depends on the facility type, the earthing system (TN, TT, etc.) and the protection devices used — there's no single universal number, which is why a correct assessment requires measurement and engineering calculation.

Can I pass an audit without a measurement report? No. A current, engineer-signed earthing measurement report is a standard requirement in both OHS audits and most OIZ/utility inspections.

How do I know whether my earthing system is TN or TT? This is usually documented in the facility's electrical project or in the connection agreement with the utility. If it's unclear, the engineer can determine it on site by examining how the transformer's neutral connection and the facility's own earthing arrangement are set up.

Does a post-incident measurement replace the normal periodic measurement? No, the two serve different purposes. A post-incident measurement documents the cause and the condition at the time of the event; the periodic measurement schedule should continue regardless of the incident.

Do weather conditions (rain, drought) affect the measurement result? Yes, soil moisture content directly affects the measured resistance. That's why the engineer notes the weather and soil conditions at the time of measurement in the report; a result taken during an unusually dry period may be re-evaluated in a different season if needed.