Of the electrical hazards found on many sites, earthing is the one that is skipped most easily. It has no presence at all while nothing is wrong, and when something does go wrong it is often the most serious kind. A leakage current on the machine enclosure is supposed to be carried away by the earthing; once the earthing fails, that protective chain is broken.
Four indications that the earthing has failed
- The enclosure is live: the machine enclosure gives a slight tingling feeling to the touch, or the test pen or the neon lamp lights up when it is applied to it. Note that without a reliable earth this indication may be unreliable, so the power must be disconnected and the result confirmed with an instrument before any conclusion is drawn.
- The residual current device trips: it trips as soon as the circuit is closed and repeated resetting has no effect. This usually means that a leakage path already exists in the insulation and the insecure earthing is only what makes the protective device operate repeatedly.
- Instrument readings drift or are interfered with: the vacuum gauge, the pressure gauge and the temperature control display show irregular movement, and in severe cases the readings are not zero even with the machine disconnected from the power.
- The machine makes an abnormal noise or the relay chatters in operation: the appearance is similar to that of an interference signal, but the root cause is a drift in the earth potential.
Three things a correct earthing has to satisfy
First, the earth electrode has to be reliable. The earth electrode is a metal part buried in the ground, and the usual practice is to drive a vertical earth rod or to install a horizontal earth electrode. The burial depth has to be sufficient, clear of the concrete foundation of the machine, and the soil should be relatively damp; several machines on the same site may share an earthing grid, but the connections must be sound.
Second, the connection has to be reliable. The earth conductor is copper strip or flexible copper cable of adequate cross-section, bolted to the electrode with anti-loosening measures, and the contact surfaces are scraped free of oxide and given corrosion protection. The earth terminals of the machine enclosures must not be connected in series to borrow the earth of an adjacent machine, and water pipes or steel structures must not be used in place of a dedicated earth conductor.
Third, the earth resistance has to meet the requirement. The limit follows the earthing arrangement of the supply system and the requirements of the local power supply authority; the site should not set its own standard. For the measurement, use a dedicated earth resistance tester and place the auxiliary earth electrode as the instrument instructions describe. The machine power supply must be disconnected before the measurement, otherwise what is measured is not the earth resistance but the value under energised conditions.
Why the earthing has to be confirmed again after the machine has been moved
This is the point at which earthing faults occur most often, for four reasons. When the machine is dragged on its castors, the cable can be crushed under a wheel or scraped by a door frame, and once the insulation is broken it may come into contact with the enclosure. The feet or the castors can crush the existing earth conductor, and the earth circuit is broken where it cannot be seen. Site works can cut through the buried electrode, or a raised floor level can leave the electrode too shallow. Where the machine was previously connected to a fixed socket and is moved to a temporary supply or is powered through a plug, the protective conductor is often not connected at all.
So after every relocation, every re-connection and every change of supply socket, the confirmation has to be carried out again.
| Symptom | What it points to | How to confirm | Action |
|---|---|---|---|
| The enclosure gives a tingling feeling or the test pen lights up | Earthing failure or damaged insulation | With the power disconnected, re-measure the insulation of the enclosure and the continuity of the earth | Check the earth conductor connection and the cable for damage, and restore the supply only after the repair |
| The residual current device trips frequently | Insulation leakage with no earth path to carry it away | Disconnect the loads in sections to identify the circuit concerned | Investigate the insulation of that circuit and try again after the repair |
| Instrument readings drift irregularly | Interference caused by a drift in the earth potential | Re-check the earth connection and the earth resistance | Redo the earthing and earth the instruments at a single point |
| The problem appears for the first time after the machine has been moved | The earth conductor has been crushed or not connected | Check along the cable route for damage and check the earth terminal | Lay the earth conductor again and measure the earth resistance |
Safety requirements
Earthing has to be checked with the power disconnected and the absence of voltage confirmed. Insulation tests are carried out with all electronic components disconnected, otherwise the control board is damaged. Where the enclosure is suspected to be live, do not confirm it by touching it with a bare hand; disconnect the power and measure with an instrument. The earth conductor must not be made up temporarily with an ad hoc wire, because a temporary earth is usually broken as soon as the machine is moved.
Earthing is not a piece of engineering that is finished once; it is something that has to be confirmed again after every relocation.