LVDC Distribution and energy efficiency.
The IEE Wiring Regulations (BS 7671:2008) are based on European Standards, which in turn are usually based on international standards.
The UK participates in both European and international standards work. Two new areas of possible development within international standards are requirements for Low Voltage DC distribution and to integrate requirements for energy efficiency into IEC 60364.
The rationale for LVDC distribution
More and more electronic equipment is being introduced in buildings which use dc.
There is also a wide range of micro generation technologies – including solar photovoltaic (PV) and wind turbines – being installed which generate dc. One of the main reasons for the proposal to introduce LVDC distribution in a building is to improve energy efficiency by reducing losses in the conversion of ac to dc for electronic loads and conversion of the dc output from micro generation to ac for mains distribution.
The challenges
There are a number of challenges when designing a LVDC installation. Persons involved in dc installations need to have the necessary expertise. Electrical equipment used on a dc installation must be suitable for direct voltage and direct current.
Equipment approved to normal ac standards may not be suitable, especially switchgear. For example, the use of plugs and socket outlets for use on dc need careful selection depending on the current rating.
Given the nature of dc, additional requirements need to be taken into account when disconnecting a dc load by withdrawing a plug from a socket outlet. This is because an arc can occur when disconnecting a load, which is more difficult to extinguish compared with an ac load
because there is no natural zero point on dc compared to ac. It is understood that one possibility being considered is to use a switched socket outlet with a plug that is interlocked
with the socket outlet. The plug and socket outlet is then designed in such a way that the plug cannot be withdrawn from the socket outlet while the plug contacts carry current.
Arc quenching
Circuit breakers for overcurrent protection is another area that needs special consideration. The arc produced when disconnecting a fault on a dc installation is more difficult to extinguish. Designers of dc installations will need to liaise with manufacturers of equipment and exercise careful consideration when selecting a circuit breaker for use on dc to ensure that the circuit breaker has suitable arc-quenching capabilities and are suitable for the operating voltage.
Cables for use on dc again need special consideration. A cable is given a voltage rating which indicates the maximum circuit voltage for which it is designed, not necessarily the voltage at which it will be used. For example, a cable designated 600/1000V is suitable for a circuit operating at 600Vac phase to earth and 1000Vac phase to phase. This cable is traditionally used in areas where mechanical strength is required such as industrial installations. For light
industrial circuits operating at 230/400Vac it would be normal to use cables at 450/750Vac, and for domestic circuits operating at 230/400Vac, cable rated at 300/500Vac would often be used. The traditional rating of the cable 300/500V is the ac rating of the cable. The dc rating of this cable for core to earth is 300×1.5 (450V dc max) and the core to core voltage is 500×1.5 (750V dc max). Therefore, designers of dc installations need to give careful consideration when
selecting a cable for use on dc to ensure it is suitable for the operating voltage and are recommended to seek advice from the manufacturer.
Renewable sources of electricity
There are a wide range of microgeneration technologies including: solar photovoltaic (PV), wind turbines, small scale hydro and micro CHP (Combined heat and power). For example, in the UK, the 17th edition of the IEE Wiring Regulations (BS 7671:2008) introduced many new requirements to ensure the safe connection of low-voltage generating sets including small scale embedded generators and solar photovoltaic (PV) power supply systems.
To view the full article from the IET's Wiring Matters Summer 2010, click on the attachment below.