In industrial installations, the protection strategy cannot be standardised at project level. The same protective measure that is mandatory in one facility may be inapplicable in the next, and the determining criterion is not installation preference but the characteristics of the connected loads and the classification of the operating environment.
1. Loads incompatible with residual current protection
The first departure from residential and commercial practice concerns equipment which, correctly installed and free of any fault, produces earth leakage currents sufficient to operate a residual current device of conventional sensitivity.
The source is constructional: the EMC filters at the input of variable speed drives, switch-mode power supplies and UPS equipment contain capacitors connected to the protective conductor. The current flowing through them is not a fault but a normal operating condition. A drive of medium rating can generate leakage currents in the tens of milliamperes. Several such loads on the same circuit exceed the threshold of a 30 mA device with no fault present.
The second consideration concerns the waveform of the fault current. A variable speed drive does not produce a sinusoidal earth fault current. A Type AC residual current device does not detect it correctly, and a Type A detects it only partially. Circuits incorporating drives require Type B, which detects the direct current component. A device of unsuitable type installed on such a circuit may fail to operate when a fault occurs — a condition more adverse than the absence of protection, since the installation is presumed protected.
Compensating measures
Where residual current protection cannot be used on a circuit, the protective function is achieved by other means:
- Earthing and bonding sized accordingly. The governing criterion is the earth fault loop impedance, which must secure operation of the short-circuit protection within the prescribed disconnection time. The value is established by measurement, not by estimation.
- Circuit segmentation. Residual current protection on small groups, rather than a single device serving the whole board, keeps cumulative leakage below the operating threshold and preserves discrimination.
- Adapted sensitivity. Devices of 100 or 300 mA on machine circuits for fire protection, with protection of persons secured by other means.
- IT system with permanent insulation monitoring, in applications where continuity of supply is critical. The first fault is signalled without interrupting operation and must be rectified before a second occurs.
- Galvanic separation by transformer, isolating the section of installation generating the leakage current.
2. Explosive atmospheres and fuse gear
In areas with a potentially explosive atmosphere — paint shops, installations handling combustible dust, fuel dispensing sites, areas with solvent vapours — conventional modular gear ceases to be the default solution.
A modular circuit breaker interrupts the arc in an arc chute that vents to the surrounding environment. Under normal operating conditions this behaviour is acceptable. In an atmosphere containing combustible dust or flammable vapour, the same characteristic becomes a source of risk.
Under these conditions fuse gear is used: the D system (Diazed, sizes DI, DII and DIII) and the compact Neozed variant. The fuse element is embedded in quartz sand; on rupture the arc is extinguished within the ceramic body and the energy is absorbed by the filler, with no emission to the surroundings. The same characteristics justify their use where the prospective short-circuit current is high, the breaking capacity of a fuse link exceeding that of conventional modular gear.
The operational limitations are nevertheless real: the element is replaced rather than reset; a stock of fuse links in the correct ratings is required; and the maintenance procedure must preclude substitution with a higher-rated link. This last condition negates the protective function entirely and is encountered frequently in service.
Area classification precedes board design. The selection of equipment and the ingress protection rating of enclosures follow from that classification, not from installation preference.
3. Data required for sizing
An industrial board is not sized according to floor area or contracted capacity, but according to the loads connected to it. For each significant load the following data is required:
- Rated power and current, and in particular the starting current and its duration. A direct-on-line motor may draw six to eight times rated current for several seconds. Protection selected solely on rated current will operate at every start.
- Duty cycle: continuous, intermittent, or with frequent starting. Starting frequency determines the duty imposed on the switching devices.
- Manufacturer's declared leakage current, for the reason set out in section 1.
- Load type: resistive, inductive or non-linear. This determines the tripping curve (B, C or D) and the level of harmonic content introduced into the installation.
- The equipment's own protective devices, with which upstream protection must discriminate.
Consequence of proceeding without this data: repeated operation in the absence of a genuine fault. The remedy commonly applied in such cases — increasing the rating of the protective device until operation ceases — removes protection of the conductor. The installation continues to function solely because the fault the device was intended to address has not yet occurred.
For this reason, industrial work begins with measurement and with review of the equipment documentation, in advance of quotation. Declared load and measured peak demand differ in most cases, and for loads with heavy starting duty the difference governs the sizing.
4. Criteria for assessing the installation
The quality of an industrial board is not assessed by the components specified, but against the following criteria: each protective device corresponds to the load it serves; the schematic exists and matches the physical configuration in the enclosure; conductors and terminals are marked such that later intervention does not require reconstructing the schematic; and the constructional solution accounts for the operating environment — temperature, dust, humidity, vibration, area classification.
Spare capacity, internal layout and marking quality become relevant at the first extension or the first remedial intervention.
Related services
Repeated operation with no identified fault
We measure actual loads and leakage currents, verify the type and rating of the installed devices, and establish whether the source is in the board or in the equipment.