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.
- Circuit breakers are sometimes removed from panels and an RCD used in their place. This is incorrect. An RCD is used as supplementary protection alongside the appropriate circuit-protection measures.
- The article advises against using 30 mA Type AC RCDs. Their operation must not be tested by deliberately creating an uncontrolled fault on equipment enclosures. B- and C-curve miniature circuit breakers require proper coordination when used in series; a Type C MCB should not be treated as a deliberately time-delayed device.
- 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.
- The article identifies confusion between TT and TN earthing arrangements arising from the translation of the German VDE 0100 rules into Turkish earthing guidance, leading to incorrect installation calculations.
- 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 recommended maximum resistance values are below 1 Ω for protective conductors and below 0.1 Ω for equipotential bonding conductors.
Kind regards,