The technical characteristics of an RCD are critical to their performance, safety, and reliability. By having a greater understanding of the importance of each of these characteristics will provide you with valuable insight to why it is important to use the right RCD for the right application, with varying types of RCDs available on the market can be installed into your electrical system.
Sensitivity & Tripping Time
The sensitivity, more formally known as the Rated Residual Operating Current I∆n, is the earth leakage current that will cause the RCD to trip. The sensitivity most commonly used in Australia is 30mA for protection of people from death by electric shock, however other common ratings are 10mA for hospital applications and 100mA & 300mA for mining applications.
A 30mA device must not trip at 15mA (½ x I∆n) and must trip at 30mA (1 x I∆n). Most manufacturers aim for the middle of this range, and the actual tripping current is approximately 22mA.
A 30mA device when tested at 30mA is allowed by the standard to take up to 300ms to trip. It is a common misconception that this value is 30ms or 40ms, which is not the case. Whereas a 10mA device must trip within 40ms regardless of the test current. The trip values are given in Table 2 (Clause 5.3.8) of AS/NZS61009.1
Types
As well as having different sensitivity in leakage current, RCDs can have different sensitivity to different waveform shapes. These are defined as different types, with Type AC, Type A, Type F and Type B being defined in Clause 2.6.2.2.1 of AS/NZS3000:2018.
Type AC have traditionally been used for most general light and power applications, but as more and more loads have electronics built in there is a shift towards RCDs that are more suitable for the waveforms created by these electronics.
On 30th April 2021, AS/NZS3000:2018 Amendment No.2 was released. That amendment included a clause relating to Type AC & Type A RCDs;
Clause 2.6.2.2.2 starting at Paragraph 4;
RCDs shall be of the type for which tripping is ensured –
(a) for residual sinusoidal alternating current; or
(b) for residual sinusoidal alternating current and residual pulsating direct current.
For 24 months from the date of publication of Amendment No.2, either Item (a) or (b) may apply. Following that time Type AC RCDs as described in Item (a) shall not be installed.
Current Rating and Breaking Capacity
For devices that don’t provide overload protections such as RCCBs and Add-on RCDs, the current rating is the maximum current that is allowed to flow through the device. These types of RCDs must be paired with an upstream overload protection device. Likewise RCDs have a conditional Breaking Capacity based on coordination with an upstream device clearing the short circuit.
RCBOs provide their own overload and short circuit protection and so will have values in line with the overload current that will cause tripping, and the maximum short current that can be cleared.
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