Primer on RCD basics
Friday, 22 May, 2009
What is the difference between a circuit breaker and an RCD?
A normal circuit breaker or fuse protects cables and other distribution components from the damaging effects of overloads and short circuits. However, it is not sensitive enough to protect a person who may accidentally come into contact with a live conductor and receive an electric shock. A residual current device (RCD) is especially designed to trip at a time and current low enough to prevent a person from receiving a permanent injury or death from an electric shock should the primary means of protection, normally insulation, fail.
What is the tripping current of an RCD?
An RCD can trip as low as half its rated sensitivity and must trip at its rated sensitivity. Most RCDs used in Australia are 30 mA type, so these can trip as low as 15 mA and must trip at 30 mA. This is different from other protection devices, which only trip when the value exceeds the set point.
What is the maximum allowable tripping time?
A common misconception is that RCDs must trip in 30 ms. In fact, an RCD when tested at its rated sensitivity must trip in 300 ms. When tested at five times, ie, 150 mA for a 30 mA device, it should trip in 40 ms. A 10 mA device must always trip within 40 ms regardless of the test current.
Changes to the Wiring Rules
In the 2007 edition of AS/NZS 3000 Wiring Rules, there were many changes made, but the three main changes which affect RCDs and the way that they are used are listed below. It is important to remember that the main purpose of increasing the usage of RCDs is to make installations safer.
1) Clause 126.96.36.199. The maximum number of circuits that can be connected to any one RCD is now three. Also, domestic installations with more than one final sub-circuit must have a minimum of two RCDs. The aim of this change is prevent a single earth leakage fault from dropping out an entire installation. And, by splitting the loads it reduces the background leakage caused by computer and other electronic devices and reduces the chance of nuisance tripping.
2) Clause 2.6.1 Note 3. This is not a requirement but a suggestion to consider 10 mA RCDs in areas of increased risk such as bathrooms or kindergartens. As 10 mA RCDs trip at a lower current and in a faster time, they therefore reduce the potentially hazardous effects of a shock on sensitive people, such as children or the elderly. When installing 10 mA RCDs, care must be taken to separate the circuits, especially those with electronic loads, as the increased sensitivity also means an increased chance of nuisance tripping.
3) Clause 188.8.131.52. All lighting and GPO circuits rated less than 20 A must now be protected by an RCD. GPO and lighting circuits are the most common place for a person to come into contact with a live conductor. These circuits were previously protected in domestic installations, but must now be protected in all commercial and industrial applications as well. This change will make these installations safer, especially for people who may not be familiar with the dangers of electricity.
A computer can have an earth leakage current of up to 1.5 mA, which means the recommended maximum number of computers that can be connected to a single RCD circuit is 10. Note that a computer does not need to be faulty to have a leakage current, indeed a brand new computer straight out of the box can register a leakage current. This is because there is a noise filter on the power supply input that diverts the electrical noise to earth.
For electronic devices other than computers, the same principles would apply, but the leakage currents of these devices may vary and will need to be checked before determining the maximum number that can be connected onto one circuit. Even if the earth leakage generated by electronic devices is not high enough to cause the RCD to trip, it will increase the amount of background leakage, which may cause a trip on a leakage current that is smaller than what it would have needed to be if there was no background leakage.
An electronic ballast for lighting can have an earth leakage current of 0.3 to 0.5 mA per ballast. For this reason, the maximum number of lights with ballasts per 30 mA RCD would be 30.
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