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Auxiliary circuits requirements
from Connections - Spring 2015
by NICEIC
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An introduction to the requirements for auxiliary circuits
This article is the first of two dealing with auxiliary circuits. Whereas this article aims to provide a broad overview of use of auxiliary circuits, the second article will focus on the particular requirements of auxiliary circuits used in conjunction with instrument transformers.
Although Section 557 – Auxiliary Circuits is introduced by Amendment 3 of BS 7671 as a separate section, the topic is not completely new. In previous editions of BS 7671, auxiliary circuits were referred to as ‘control circuits’ and were the subject of Regulation 537.5.3 (now deleted).
Part 2 of BS 7671 defi nes auxiliary circuits as circuits for the transmission of signals intended for the detection, supervision or control of the functional status of a main circuit, such as circuits for control, signalling and measurement. The circuits used by a building management system (BMS) to monitor temperature and humidity at various points within a building are an example of auxiliary circuits.
Regulation 557.1 makes it clear that Section 557 only applies to auxiliary circuits not covered by other product or system standards such as the construction of assemblies of electrical equipment to the appropriate part of the IEC 61439 series. However, as with many other standards, such as BS 5839 dealing with Fire Detection and Alarm Systems, it is a requirement of the standard that the installation of wiring associated with the alarm system complies with the requirements of BS 7671.
Rather than being concerned with the operational characteristics of the equipment supplied by the auxiliary circuit, Section 557 is concerned with ensuring that the power supply is safe, reliable and does not impact upon the functionality of the equipment supplied by the auxiliary circuit. Broadly speaking, the contents of Section 557 cover the following areas: 1. The electrical supply to the auxiliary circuit and the associated protective measures (see Regulation Group 557.3). 2. The minimum cross-sectional area of the cables used for the auxiliary circuit (see Regulation Group 557.4). 3. The special requirements of auxiliary circuits used for measurement (see Regulation Group 557.5). 4. Measures required to avoid loss of functionality in the auxiliary circuit (see Regulation Group 557.6).
Where the auxiliary circuit is a control circuit, Regulation 557.3.201 requires that the circuit should be designed, arranged and protected so that a fault between the circuit and other conductive parts is not liable to cause the equipment being controlled to malfunction, i.e. not operate as intended.
The auxiliary circuits described in Section 557 may operate using an a.c. or d.c. supply which can either be obtained by either a connection to the main power circuit or from an independent source such as a battery. Section 557 includes requirements for the electrical protection provided for auxiliary circuits and the method used will depend upon whether the auxiliary circuits are earthed or unearthed.
Rather than being permanently connected to the power supply some auxiliary circuits are connected via a switching device and Regulation 557.6.5.1 contains requirements for auxiliary circuits without a direct connection to a main circuit.
Power supplies for auxiliary circuits dependent on the main a.c. circuit are detailed in Regulation 557.3.2.1. The recognised methods of connecting the auxiliary circuit are: 1. direct connection to the main a.c. supply (as shown by Fig 1), 2. via a rectifi er (as shown by Fig 2), and 3. via a transformer providing simple separation between the main and auxiliary circuits (as shown by Fig 3).
Regulation 557.3.2.1 recommends that where auxiliary circuits primarily supply electronic equipment or systems, then rather than being supplied directly (as shown in Fig 1) there should be, at least, simple separation from the main circuit. It should be noted, however, that rather than being a requirement this is a recommendation.
Auxiliary circuits do not necessarily operate at extra-low voltages and the maximum preferred values of the nominal voltage for auxiliary circuits used for control purposes is as shown in Table 1 (Regulations 557.3.5.3 and 557.3.5.4.1 refer):
Fig 2 Auxiliary circuit supplied from
the main circuit via a rectifier
Fig 3 Auxiliary circuit supplied from
the main circuit via a transformer
Table 1 Maximum preferred values of
nominal voltages of auxiliary circuits
Nominal frequency Maximum nominal voltage (volts)
Terminals for the auxiliary circuit
Terminals for the auxiliary circuit
Terminals for the auxiliary circuit d.c. 220
a.c. (50 Hz) a.c. (60 Hz) 230 277
Rather than being supplied from the main circuit, Regulation 557.3.3 permits auxiliary circuits to be supplied by an independent supply which could be a battery. However, the independent auxiliary circuit should not be able to create a hazardous situation. Where the main circuit suff ers a loss of supply or an undervoltage occurs, this should be detected.
Where an auxiliary circuit, connected to the main circuit via a transformer, requires a connection to earth, Regulation 557.3.4.2 requires that there should only be one point of connection which should be located on the secondary side, situated close to the transformer, be easily accessible and, for ease of carrying out insulation measurement, be capable of being isolated.
Regulation Group 557.3.6 contains two regulations dealing with protective measures. The fi rst Regulation 557.3.6.1 contains requirements aimed at protecting the wiring systems. For single-phase earthed a.c. or d.c. auxiliary circuits supplied by a transformer, as shown by Fig 4, it is permitted to protect the auxiliary circuit by protective devices inserted in the conductors not connected directly to earth.
In unearthed auxiliary circuits, Regulation 557.3.6.1 requires that short-circuit protection shall be provided for all line conductors (see Fig 5). As an alternative to the arrangement shown, the short-circuit protective devices can be connected in the primary winding.
In order to ensure adequate mechanical strength in BS 7671, Table 557.1 provides minimum values of cross-sectional area for copper conductors used in auxiliary circuits (see Table 2).
Regulation group 557.5 contains guidance on using auxiliary circuits in measurement systems where the auxiliary circuit may either be directly connected to the means of measurement (557.5.2) or connected via a transformer (557.5.3). When measuring or monitoring voltages or currents in high voltage/high current circuits such as those
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Fig 4 Position of protective devices in an auxiliary circuit
connected to Earth and supplied by a transformer
Fig 5 Devices providing short-circuit protection in an auxiliary circuit NOT connected to Earth and supplied by a transformer
found in electrical distribution systems, it is desirable to use instrument transformers to provide electrical separation between the high voltage/high current electrical distribution system and the auxiliary circuits used for measurement. The use of auxiliary circuits in applications utilising instrument transformers will be subject of a following article.
Auxiliary circuits oft en form an integral part of control and instrumentation systems where the system needs to be robust, reliable and not subject to malfunctions. Regulation Group 557.6 provides the designer and installer of auxiliary circuits with guidance on measures intended to ensure that, when installed, the circuits function as intended with a high degree of reliability. The regulations included in Regulation Group 557.6 include requirements for: • The voltage supply (557.6.1) • Quality of signals depending upon cable characteristics (557.6.2) • Measures to avoid loss of functionality (557.6.3) • Current-limiting signal outputs (557.6.4) • Connection to the main circuit (557.6.5)
Application Type of cable Single-core or Two-core Multicore Stranded Screened Screened or unscreened
Control circuits 0.5 Data transfer –0.5 0.1 0.1
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