I.S. 10101 / ET101 Wiring Rules cheat sheet

The wiring rules apply to electrical installations in buildings and to fixed installations supplied at standard low voltage; they cover design, erection, inspection and testing.

Fundamental principles: protection against shock, thermal effects, overcurrent, fault current and surge; installations must be safe in normal use and under single-fault conditions.

Definitions: an installation is an assembly of associated electrical equipment with coordinated characteristics to fulfil a specific purpose.

A live part is any conductor or conductive part intended to be energised in normal use, including a neutral conductor but not a PEN conductor.

Basic protection prevents contact with live parts under normal conditions; fault protection protects against contact under single-fault conditions.

An exposed-conductive-part is a conductive part of equipment that can be touched and is not live in normal use but may become live under fault conditions.

An extraneous-conductive-part is a conductive part not forming part of the installation that may introduce a potential, typically earth potential.

Equipment must be selected to withstand the prospective fault current and the environmental conditions of its location.

Assessment covers: maximum demand and diversity, earthing arrangement, prospective fault current, and the nature of supply and load.

Diversity may be applied to maximum demand only where the load pattern justifies it; the designer must document the assumed diversity factors.

Installations are classified by earthing arrangement: TN-S, TN-C-S, TT, and IT systems determine protective measures and disconnection times.

Protective device selection depends on the supply characteristics: prospective short-circuit current, earth fault loop impedance, and disconnection time limits.

Conductor cross-sectional area is selected for current-carrying capacity, voltage drop, thermal withstand, and mechanical strength.

Voltage drop from the origin to the load should not exceed the limits set for the installation; lighting and power circuits are assessed separately.

The rated current of the supply and the number of circuits determine the arrangement of distribution boards and protective devices.

Environmental conditions (ambient temperature, grouping, installation method) derate cable current-carrying capacity via correction factors.

Basic protection by insulation, barriers or enclosures, or by placing live parts out of reach; fault protection by automatic disconnection, double insulation, or separation.

Automatic disconnection: the disconnection time depends on the system (TN or TT) and the nominal voltage; typical limits are 0.4 s for TN final circuits up to 32 A.

Earth fault loop impedance must satisfy Zs ≤ U0 / Ia, where Ia is the current that causes the protective device to operate within the required time.

In TT systems, an RCD is generally required because the loop impedance is too high for overcurrent devices to disconnect within the time limit.

For a TT system, the relationship Ra × IΔn ≤ 50 V applies, where Ra is the earth electrode resistance and IΔn the RCD residual current.

Supplementary protection by 30 mA RCDs is required for socket-outlets up to 32 A for general use and in certain special locations.

Protection by electrical separation uses an isolating transformer or equivalent source supplying one item of equipment only.

SELV and PELV systems limit voltage to a safe value; SELV has no earth connection, PELV may be earthed.

Overcurrent protection requires Ib ≤ In ≤ Iz, where Ib is design current, In the device rating and Iz the cable capacity.

The device must also satisfy I2 ≤ 1.45 × Iz, where I2 is the current ensuring effective operation of the protective device.

Coordination between conductors and devices ensures the cable is protected against overload and short circuit.

Short-circuit protection: the device must interrupt the prospective fault current and the cable must withstand the let-through energy (k²S² ≥ I²t).

Thermal effects: equipment and cables must not exceed their temperature limits in normal use or under fault conditions.

Surge protective devices (SPDs) limit transient overvoltages; they are selected by voltage protection level, discharge current and location.

SPDs are installed at the origin and, where required, near sensitive equipment; coordination between SPDs uses their voltage/current characteristics.

Overload protection may be omitted where interruption of the circuit would create a hazard, provided the risk of overload is excluded.

Wiring system selection considers: environmental conditions, mechanical stress, current-carrying capacity, and ease of maintenance.

Cables must be supported at intervals appropriate to the cable type and protected against mechanical damage where exposed.

Cable installation methods (clipped direct, in conduit, in trunking, buried) determine the reference method and hence the current rating.

Grouping of cables and thermal insulation reduce current-carrying capacity and require correction factors.

Segregation is required between power and communication/telecommunication cables to limit interference and fault spread.

Fire safety: wiring systems in escape routes and fire compartments must maintain circuit integrity where required.

Joints and connections must be accessible unless they are of a type that is maintenance-free and suitable for the location.

Cable entries into equipment must maintain the degree of protection (IP rating) of the enclosure.

Conductor material is copper or aluminium; cross-sectional areas are selected for current capacity, voltage drop, and thermal withstand.

Minimum cross-sectional areas apply for fixed wiring and for flexible cords depending on the circuit and protective conductor function.

The neutral conductor must have a cross-sectional area not less than that required for the load; it may be reduced only where harmonics are negligible.

The protective conductor size depends on the phase conductor size, using the standard table relationship (S ≤ 16 mm² → S; larger sizes use S/2 with a minimum).

The adiabatic equation S ≥ √(I²t)/k verifies the protective conductor withstands fault energy, with k depending on conductor and insulation material.

Cable insulation types (PVC, XLPE) determine the maximum conductor operating temperature and hence the current rating.

Voltage drop is calculated from the load current, cable resistance and reactance, and circuit length; it must remain within the installation limit.

Conductors must be identified throughout the installation by colour or marking consistent with their function.

Earthing arrangements: TN-S uses a separate protective conductor; TN-C-S uses a combined PEN split at the origin; TT uses a local earth electrode; IT is unearthed or earthed through impedance.

The main earthing terminal (MET) connects protective conductors, main protective bonding conductors, and the earthing conductor.

Main protective bonding connects extraneous-conductive-parts (water, gas, structural steel) to the MET; the conductor size depends on the supply neutral size.

Supplementary bonding connects simultaneously accessible exposed and extraneous-conductive-parts locally where disconnection times cannot be met.

The earthing conductor size is selected from the phase conductor size and must withstand prospective fault current.

The earth electrode resistance in a TT system must satisfy Ra × IΔn ≤ 50 V for the RCD protecting the installation.

Protective conductors must be identified green/yellow and must not carry load current except in a PEN conductor.

Bonding conductors must be mechanically protected and connected by permanent, reliable means.

Every installation must have a means of isolation and switching for maintenance, emergency, and functional control.

Isolation must be by a device that provides a visible or clearly indicated break and is lockable or otherwise secured against reconnection.

Switching devices must be selected for the rated voltage, current, frequency, and the prospective fault current at the point of installation.

Devices must be capable of making and breaking the load current and, where required, the short-circuit current.

Emergency switching must be provided where a hazard could arise and must be clearly identified and accessible.

Functional switching controls the load in normal use; it need not provide isolation.

Protective devices must be coordinated so that the device nearest the fault operates first (discrimination).

RCDs must be selected for the type of residual current (AC, pulsating DC, smooth DC) and the required sensitivity.

Bathrooms: equipment is zoned; socket-outlets are generally excluded from zones 0, 1 and 2, and supplementary bonding may be required.

Zone 0 is inside the bath or shower basin; zone 1 is above it; zone 2 extends beyond zone 1; equipment must suit the zone.

Construction sites: installations must resist mechanical damage and harsh conditions; 30 mA RCDs protect socket-outlets and reduced voltages are used where required.

Agricultural and horticultural premises: equipment must withstand dust, moisture and corrosive atmospheres; RCD protection is widely required.

In locations with fire risk, equipment must be selected and installed to limit ignition and fire spread.

In conductive locations (metallic enclosures, restricted movement), protection by SELV, PELV, or electrical separation may be required.

Equipment in special locations must have an appropriate IP rating for water and dust ingress.

Where a special location requires a measure not used elsewhere, the requirement is additional to the general rules.

Verification comprises inspection, testing, and the issue of a certificate and schedule of test results.

Inspection before testing checks: correct selection and erection, protection measures, identification, and accessibility.

Continuity testing verifies protective conductors, ring final circuits, and main and supplementary bonding.

Insulation resistance is measured with the installation isolated; the result must meet the minimum value for the circuit voltage.

Earth fault loop impedance is measured to confirm that protective devices disconnect within the required time.

RCD testing verifies the tripping current and operating time at the rated residual current and at half and five times that value.

Polarity must be verified so that all switches and protective devices are in the phase conductor.

Test results are recorded on a schedule, and the certificate confirms the installation complies with the wiring rules.