Paul Chaffers, Technical Events Manager and Technical Author of NAPIT On-site Solutions, takes a closer look at the requirements for automatic disconnection in case of a fault.
Introduction
BS 7671 Chapter 41 deals with protection against electric shock, and this is where the requirements for automatic disconnection in case of a fault can be found.
The disconnection times stated in Table 41.1 are influenced by the type of earthing system and the voltage range used. Also, RCDs are included within the design for fault protection for certain installations, meaning there are many different parameters to be met.
Automatic Disconnection of Supply (ADS)
In general, there are two aspects involved with this protective measure:
– Basic protection is used to prevent contact with live parts, and
– Fault protection is provided by the protective earthing system and automatic disconnection in case of a fault.
Fault to Earth
Fig 1 relates to TN systems and shows the path of the Earth fault current in the event of line to Earth fault in one of the circuits.
For clarity, only one circuit is shown, and it supplies a load with a metal casing that is connected to the Earth via the circuit protective conductor (CPC). The example shown is for a TN-S earthing system.
The direction of the Earth fault current path is indicated by the arrows in Fig 1. It will be observed that the fault current flows in a loop. It returns back to the supply transformer via the protective conductors, then goes through the winding of the transformer and returns to the fault within the installation via the line conductor.
For TT systems, refer to Fig 2, where the mass of Earth between the installation and the origin forms part of the Earth fault current path.
The resistance met by the fault current is known as impedance due to the presence of the transformer winding in the path, and the symbol for impedance is Z. This fault path is called the Earth fault loop impedance and is abbreviated to Zs.
The unit of impedance is the same as the unit of resistance, which is an ohm. The intention is that the fault current will be high enough to disconnect the overcurrent protective device within the required time, known as the disconnection time.
The value of the Earth fault current is directly related to the value of operating voltage and Earth fault loop impedance, known simply as Ohmβs Law.
Therefore, the lower the value of Zs, the higher the value of the Earth fault current and the quicker it will operate the protective device in order to disconnect the faulty circuit.
Disconnection times
As the severity of an electric shock depends not only on the value of the current flowing but also upon the time that the current flows, BS 7671 quotes the maximum disconnection times for each type and rating of overcurrent protective device. This means that the protective device of the circuit must operate to disconnect the electricity supply within these times in the event of a fault to Earth.
Regulation 411.3.2.2 states that disconnection times in Table 41.1, found in BS 7671, shall be applied to final circuits with a rated current not exceeding:
– 63 A with one or more socket-outlets, and
– 32 A supplying only fixed connected current-using equipment.
The disconnection times for 230 V AC final circuits, found in Table 41.1, are:
– 0.4 s for TN systems
– 0.2 s for TT systems.
You should be aware that when disconnection is achieved in a TT system by an overcurrent protective device and protective equipotential bonding of all extraneous-conductive-parts has been carried out, in accordance with Regulation 411.3.1.2, the maximum disconnection times for TN systems may be used.
In distribution circuits, i.e., sub-mains, and for final circuits not covered by Regulation 411.3.2.2, Table 41.1 does not apply. In TN systems, the disconnection time must not exceed 5 s; in TT systems, the disconnection time must not exceed 1 s (see Regulations 411.3.2.3 and 411.3.2.4).
When the Earth fault loop impedance, Zs, is measured using an Earth fault loop impedance tester, the reading obtained must be less than the relevant maximum value of Zs in the appropriate table. Maximum Zs values are given in BS 7671 Tables 41.2, 41.3 and 41.4. The headings of these tables must be carefully noted to understand how each table is to be used.
Zs corrected for temperature
It should be noted that BS 7671 Tables 41.2, 41.3 and 41.4 all have a similar note at the end of the table.
This note states that the maximum impedance given in the table should not be exceeded if the impedance is measured when the circuit conductors are at their maximum operating temperature (i.e., 70Β°C). Usually, the circuit conductors will be at much less than this when testing is carried out. They may be assumed to be 20Β°C.
Values taken at 20Β°C can be corrected to 70Β°C for 70Β°C cables by multiplying the maximum value of Zs by 0.8, as stated in Appendix 3 of BS 7671.
Maximum Zs figures for commonly used devices, corrected for temperature, are produced in Table 1.
RCDs used for fault protection
For TT Earthing systems, RCDs are often used to provide fault protection. This is mainly due to the fact that Earth electrode resistance is too excessive to meet the maximum Earth fault loop impedance requirements for the circuitβs protective device. It is important that the electrode resistance remains stable and therefore should be as low as practicable (under 200 Ξ© is deemed satisfactory). Table 41.5 of BS 7671 lists the maximum Earth fault loop impedance values to ensure effective RCD operation, as summarised in Table 2.
Conclusion
We commonly see multiple protective devices used throughout installations, with ratings for different applications. It is vital inspectors are careful to note the exact characteristics of the device to ensure the correct look-up table is used to confirm ADS can be achieved.
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