Mistakes in Electrical Installations and Earthing
Earthing is a vital part of an electrical installation because it protects users, operating personnel and other living beings from electric shock caused by dangerous touch and step voltages. Correct system sizing and installation also help prevent fire and damage.
Errors in Installations and Earthing
- UPS systems, lifts, telephone lines, lightning protection, natural gas pipework and IT installations are sometimes earthed separately. They should be considered within the central earthing and bonding system. A blanket requirement for an earth resistance of 2 or 5 ohms wherever an RCD is installed has no basis in itself; the appropriate values must be established by calculation.
- In telecommunications installations, measurements are sometimes taken between PE, the protective conductor, and N, the neutral, or at a socket, and interpreted as evidence of poor earthing. Such a result may instead reflect an unsuitable instrument or measurement method and a voltage drop on the neutral. Electromagnetic compatibility, interference currents, stray currents and harmonics must be considered.
- RCD is the general term for residual current devices. An RCCB without overcurrent protection does not replace a circuit breaker or fuse. An RCBO combines residual-current and overcurrent protection. Select the protection arrangement using the device type, circuit characteristics and manufacturer data.
- Choose the RCD type and sensitivity for the load and applicable standard; a 30 mA Type AC device is not automatically suitable for every circuit. Do not test operation by creating an uncontrolled enclosure fault: use suitable test equipment and a safe procedure. Verify selectivity and coordination between B- and C-curve MCBs. Curve C does not mean that a device is a time-delayed type.
- In TN and TT installations, protective conductors must not simply be bridged from one motor or socket to another without the required circuit arrangement. Each circuit must have its conductors routed and connected within its own sheath or conduit as applicable, to avoid transferred potentials during a fault.
- TN and TT earthing systems use different protection arrangements. When applying references such as VDE 0100, correctly match the system type, protective conditions and calculation method; requirements from different systems must not be used interchangeably.
- Selecting cables solely from operating current calculated from power can result in cross-sections 3 to 5 times larger than necessary, increasing installation costs.
- MCBs are sometimes installed in series without understanding their operating characteristics, causing the wrong breaker to trip. MCCBs may also be installed indiscriminately at incoming and outgoing points. Checking selectivity by deliberately creating an enclosure fault or short circuit is an unsafe practice.
Periodic Inspections
- General checks should cover the following items, preferably in this order:
- Measure continuity of protective conductors and main and supplementary equipotential bonding connections;
- Measure the installation's insulation resistance;
- Measure soil resistivity;
- Measure earth resistance;
- Verify automatic disconnection of the supply. The minimum single-phase short-circuit current must be calculated for final circuits;
- Verify RCD operation in terms of both tripping current and time;
- Measure and inspect continuity of protective conductors and main and supplementary equipotential bonding connections. Periodic verification of this continuity is mandatory.
- A test source providing at least 0.2 A, with an open-circuit voltage between 4 V and 24 V AC or DC, is recommended.
- The example resistance limits in this text are < 1 Ω for protective conductors and < 0.1 Ω for equipotential bonding conductors. Verify the actual acceptance criterion against conductor characteristics and the applicable standard.
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