|
|
The content of this article is written specifically in the Australian context. Power networks and practices vary from one jurisdiction to another, and the discussion here may not apply in other jurisdictions.
Wiring rules in Australia mandate the fitting of Residual Current Devices (RCD) to fixed power wiring in certain circumstances to provide a measure of protection against electrocution.
RCDs are also known as "Safety Switches", a clever marketing name to promote their use, but perhaps at the risk of encouraging complacency. Importantly, in the context of this article, are they effective on portable gen sets often used in ham radio field activities?
RCDs are a device intended to isolate supply to protected circuits, socket-outlets or electrical equipment in the event of a current flow to earth which exceeds a predetermined value.
RCDs depend on the neutral conductor being bonded to the protective earth conductor on the source side of the RCD. They will not work as intended if the source is isolated from the protective earth conductor.
 |
Fig 1 shows a simplified schematic of an RCD for a single phase final sub-circuit. L and N are the supply side active and neutral conductors, and the load is connected to the lower terminals. The magnetic flux in the toroidal core (3) is due to the difference in active and neutral currents. If sufficient current imbalance exists, the current in the secondary (2) will be sufficient to trip the switch contacts via the actuator coil (1). The design of the current transformer turns ratio and the tripping magnet determines the imbalance threshold that will operate the RCD.
If a person's body was to make a path from current from the load side active conductors to earth, an appropriate RCD would detect the small current imbalance and disconnect very quickly reducing the risk of electrocution.
Note that the RCD cannot detect a person's body conducting current between the load circuit active and neutral conductors, such current appears to the RCD as normal load circuit current and nets out in the summing detector.
Note especially the test switch (4) that is integrated into most RCDs. In this case, it creates a imbalance by passing a known current from the active conductor on the supply side to the neutral conductor on the load side. This tests the internals of the RCD, but it does not prove that the RCD will operate on a fault to earth on the load side.
The most thorough way to test an RCD in a typical installation is a tester which passes an accurately known current from active to protective earth, and that measures the response time for the worst case faults (typically the fault applied at 0° and 180° phase relationship to the supply). Follow the instructions that come with the tester.
The next best way to test an RCD in a typical installation is a tester which passes an accurately known current from active to protective earth. Follow the instructions that come with the tester.
 |
Fig 2 shows a simple tester. It has three lamps on the right hand side to show potential from L-E. L-N, and N-E. As shown, these are the correct indications for a power outlet in a normal fixed installation. The slide switch allows selection of an earth fault current (ie from line to earth) which is applied by pressing the test button.
The poorest and least comprehensive test is to use the TEST button on the RCD. Nevertheless, this test should be performed periodically to test the internals of the RCD.
The Australian standard applicable requires that the winding of a single phase generator must not be connected to protective earth unless:
Unless the generator set was manufactured with integral RCD protection, then it may be configured in a way that will not work properly with a plug in portable RCD unit.
The practice of referring to RCDs as "Safety Switches" may promote complacency so that if one is inline, users may be less cautious, thinking that the Safety Switch will save them from adverse outcome (being electrocution, death by electricity).
Portable Safety Switches (RCDs) may not provide the intended protection when plugged into a portable gen set or inverter.
A proper test of the Safety Switch (RCD) with a tester that creates a fault current to protective earth is the way to test whether the Safety Switch is effective in that application.
The test switch integrated into a Safety Switch may not be a complete and adequate test, and should not be relied upon.
| Version | Date | Description |
| 1.01 | 07/08/2011 | Initial. |
| 1.02 | ||
| 1.03 |
Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.
VK1OD on the 'net. I appreciate feedback, click on the ... in webm...@vk1od.net for my email address.
© Copyright: Owen Duffy 1995, 2012. All rights reserved. Disclaimer.