This is part four of the Hager Guide to the Commercial Installations(read more about the Type B Distribution Boards and compliance with standards, the cable entry + the isolation and switching, and Protection against fault current, use skilled persons!), dealing with: Protection against Electric Shock, the Socket Outlets, the Nuisance Tripping, Cables in walls, Protection against overvoltage, and Fire Detection and Alarm Circuits. Read / download the complete Hager Commercial Installations guide as a pdf online.
Hager Commercial Installations – Protection against electric shock
Protection against electric shock needs to be provided by offering both basic protection and fault protection. Basic protection includes the insulation of live parts and barriers or enclosures such as distribution boards. Appropriate devices or blanks must be fitted to maintain IP2X or IPXXB. If the top of the horizontal surface is readily accessible then the level of protection there should be IP4X or IPXXD. Automatic disconnection of supply will usually provide fault protection. This involves protective earthing, protective equipotential bonding and the automatic disconnection of a device if there is an earth fault. The designer will normally need to ensure co-ordination of protective devices and earth fault loop impedances so that disconnection will occur within the maximum times given in 4220.127.116.11, 418.104.22.168 or 422.214.171.124.
An additional requirement for the protection against electric shock is to specify RCDs where they are needed. 415.1.1 recognises that RCDs with a rated residual operating current up to 30mA and an operating time not exceeding 40ms at a residual current of 5 provides additional protection for ac systems if the basic or fault protection fails, or against carelessness by the end user.
Hager Commercial Installations – Socket outlets
Regulation 411.3.3 requires that an RCD not exceeding 30mA be provided for:
- Socket outlets up to 20A that are for general use by ‘ordinary persons’.
- Mobile equipment up to 32A that is for use outdoors.
One exception is permitted where the use of the socket outlet is under the supervision of someone ‘skilled’ or ‘instructed’. So, for commercial or industrial applications the designer will need to consult with the client about whether someone who is ‘skilled’ or ‘instructed’ will normally supervise the installation before deciding which socket outlets need RCD protection. Another exception is for a specific labeled/identified socket-outlet for a particular item of equipment.
Clearly ‘ordinary persons’ will use some commercial installations i.e. ‘persons who do not have the necessary knowledge to avoid the dangers from electricity.’ If this is the case then the designer/installer may decide to provide RCD protection to all socket outlets. For socket outlets used by cleaners, those in common or circulation areas, in self-catering areas or which might supply outdoor equipment, it is generally considered that RCD protection is required.
Nuisance tripping with the Hager Distribution Boards
In a commercial installation it is likely that socket outlets will supply computers, printers, copiers and other electronic equipment. This type of equipment produces small amounts of protective current. Nuisance tripping could be a problem if several of these are on one circuit protected by a 30mA RCD. The designer will need to consider this problem and may decide to reduce the number of sockets on each circuit by, for example, increasing the number of final circuits. Alternatively you can label sockets used for such equipment. This, plus the occupant/employer operational systems and health and safety policy, should ensure compliance where RCD protection is not provided.
Hager Distribution Boards – Cables in walls
It is likely that metal partitions will separate rooms in a commercial installation. If this wall has a cable inside it then the requirements of 522.6.8 will need to be met. These requirements are similar to those for socket outlets in that if there is adequate supervision by ‘skilled’ or ‘instructed’ persons then you do not need to provide additional RCD protection. If there is some doubt about this, then the designer could make the decision to apply part (v) of this regulation and provide 30mA RCD protection. This applies to all circuits, not just socket outlet circuits.
Hager Distribution Boards Electric shock protection – conclusion
More circuits need RCD protection since the introduction of the 17th Edition. In commercial distribution boards, it would be appropriate to use RCBOs for individual outgoing circuits.
Remember: In commercial distribution boards it would be appropriate to use RCBOs for individual outgoing circuits.
Protection against overvoltage
Section 443 of BS 7671 deals with the protection of electrical installations against transient overvoltages. These can be from the supply distribution system or generated by equipment. Overvoltage protection by surge protection devices (SPDs) is not generally needed for a distribution board where a suitable rated impulse withstand voltage is declared by the manufacturer. Table 44.4 in BS 7671 provides examples of various impulse categories for equipment and table 44.3 gives the corresponding minimum impulse withstand voltage. For distribution boards where the nominal voltage of the installation is 230/240V or 277/480V category III, 4kV would be appropriate. The designer or installer may choose to apply the requirements of regulation 443.2.4. This uses a risk assessment method to determine whether SPDs are required.
Fire detection and alarm circuits
Chapter 56 of BS 7671 covers fire detection and alarm circuits. Regulation 560.7.1 states that these safety services must be independent of other circuits.
This is also a requirement of BS 5859 Fire Detection and Fire Alarm Systems for Buildings. Clause 25.2 states that the mains supply to the fire alarm system should be from the load side of the main isolating device for the building and have its own isolating protective device (such as a circuit-breaker). The circuit should also be from a point in the electrical distribution system that is close to the main isolating device for the building. In addition, every protective device that can isolate the supply to the fire alarm system, other than the main isolator for the building, should be clearly labeled: “FIRE ALARM. DO NOT SWITCH OFF” in a durable and fade resistant material.
This is part 4 of the Hager Full Guide to the Commercial Installations – read the previous articles about the Type B Distribution Boards and compliance with standards, the cable entry + the isolation and switching, Protection against fault current, use skilled persons!(download the full pdf file of the guide via the Hager website) – we will continue to post the next parts of the guide, keep an eye on Blog SparksDirect].