RCD testers and appliance testers with RCD trip facilities operate by simulating a current leakage fault, and making an accurate measurement of the amount of time it took for the RCD under test to cut power. This type of a test is known as an RCD trip time test.
The trip time recorded by the RCD tester determines if the RCD under test meets the requirements of the AS/NZS3760:2010. These trip times may need to be as low as 40 milliseconds so the equipment used needs to be highly accurate.
The most frequently used RCD test (as specified in the AS/NZS3760:2010) requires a trip time test at the rated current of the RCD (ie, a fault is applied at the rated current of the RCD). The fault current must be 'suddenly applied' by the RCD tester. In most cases the RCD will be sensitive to 30mA, although in some circumstances (such as medical institutions) 10mA sensitive RCD's maybe in installed, or in some instances, variable trip current RCD's may be in use..
The AS/NZS3760:2010 standard specifies the requirements for RCD testing. Further information regarding RCD testing can also be found in Best Practices Guide to Testing and Tagging.
How an RCD tester is commonly used
While accurate devices, RCD tester are relatively simple to operate. The AS/NZS3760 specifies that users of such equipment must be deemed 'competent' and therefore the operation of RCD testers is covered by many TAFE and privately run courses offering competency in testing and tagging.
Most RCD testers have standard Australian 3 pin plug leads. This allows the RCD tester to be plugged directly into the mains power supply. From here, the switch board or power socket RCD can be tripped - usually by simply selecting the appropriate current, and simply pressing the test button.
Portable RCD's can also be checked in the same manner but may require the use of an Isolation Transformer. During the test, the RCD tester will apply the fault condition and measure the amount of time it took for the RCD to trip. Obviously, power is cut on the circuits that the RCD protects. Caution must always be exercised for this reason.
The amount of time measured by the RCD will determine if the result is a PASS or FAIL.
Help -> For more information on the tests required under the AS/NZS3760:2010 see Best Practice Guide to Testing and Tagging
RCD testers with probes or test leads
Some RCD testers are also available with probes (rather than a 3-pin plug). The probes can be used in power outlet sockets (or at the switch board) to enable the trip time measurement. This type is also frequently used to trip time test 3-phase RCD's.
These types of RCD test leads are usually only suitable for qualified electricians. It's therefore worth checking if the RCD tester you're considering is supplied with a 3-pin plug.
All RCD testers supplied by Test and Tag Supplies are provided with a 3-pin plug and in some instances are also supplied with test leads and probes.
The TTS4112EL is one of the lower cost RCD testers we sell at Test and Tag Supplies which can be used to meet the requirements of the AS/NZS3760:2010.
Switchboard testing and Category ratings
Many qualified electricians prefer to test RCD's at the switchboard rather than from power outlets. Whilst this does not necessarily check the integrity of operation of the RCD from an actual power outlet, it sometimes provides a less time consuming means of testing the operation of the RCD's.
Many dedicated RCD testers are designed for this type of functionality and are provided with appropriate test leads. However, more importantly RCD testers (and other test equipment) should have a category rating of at least CATIII for connection directly to a switchboard.
The category ratings of test instruments are determined by their suitability to be used at various locations within an installation depending on their ability to withstand electrical transients such as overvoltage and voltage spikes - in accordance with the IEC standard 61010.1 Safety requirements for electrical equipment for measurement, control, and laboratory use - Part 1: General requirements
Appliance testers with integrated RCD testing facilities are generally only CATII rated instruments and are therefore not suitable for connection directly to a switchboard. Attempting to connect an appliance tester to a switchboard directly may cause damage to the equipment, and will not provide the user with a suitable level of protection.
Current ranges
Most RCD testers have a range of tripping currents. This may range from 5mA up to 500mA and in some instances, higher. As most RCD's are designed to trip at 30mA, a 30mA setting is essential.
'1/2 trip' and 'fast trip' are also common features which allow the user to check the RCD functions under different conditions.
Tripping angle
RCD testers should also have selectable tripping angles. This basically indicates whether the current is at the 0 degree crossing point as the current is rising, or the 180 degree crossing point as the current is falling that the fault is applied. Most RCD testers have 0 and 180 degree selectable angles.
Ramp current testing
During a ramp current test, a current leakage fault of increasing value is applied.
The purpose of this test is not to measure the trip time, but to indicate at which current the RCD becomes sensitive to current leakage conditions.
Generally a 30mA rated RCD's will trip at about 75% of its rated current. If, during a ramp current test, the 30mA RCD tripped at 13mA, the RCD may be considered over sensitive and perhaps, faulty.
Although ramp current testing is not required under the AS/NZS3760:2003, it can be a useful test during fault finding.
Recording RCD testers
Some RCD testers boast recording capabilities. While initially the thought of a recording RCD tester sounds attractive, consideration must be given to whether this feature will actually be used.
On most recording appliance testers Site, Location, and description data is all recorded. This allows for the test results to be effectively understood and interpreted at the time of download to a PC.
At present, dedicated recording RCD testers do not record any such data. Usually only the time, memory position, tripping angle, current, and trip time is recorded. In the absence of any information regarding reference number, site, location or description, how useful will this data actually be?
However, the Metrel range of installation testers and RCD testers we provide do record data at various levels of an installations structure. These are generally recorded in 'Objects, Blocks, and Fuse' levels.
Integrated RCD testers
Some appliance testers such as the Metrel 3309, PAC3760DL, Seaward PT300, Megger PAT 150 and 450, Primetest Elite and Trio SmartCal SafeTcheck have integrated RCD testers. Integrated RCD testers deliver the obvious benefit of portability.
However, it is worth considering that integrated RCD testers may not provide the same level of functionality of a dedicated RCD tester. Sometimes tripping currents are fixed at pre-determined levels. It's worth considering how you'll be using the RCD tester before deciding which solution is more appropriate for you.
Low cost RCD testers
There are some very low cost devices (under $250.00) available in Australia which are sold as 'RCD testers'. However most of these low cost devices are not RCD trip time testers and will only deliver a current leakage fault to determine if the RCD trips at all. These units cannot be used to test in accordance with the AS/NZS3760.
Trip Current Electronics
RCD testers simulate a fault current by creating an imbalance between the active and neutral conductors of the supply. This fault current can be produced in a number of ways. Lower cost RCD testers may use simple resistors to create the fault current - creating a path to earth using a set value resistor. More advanced RCD testers will create the trip current electronically.
RCD testers which simply create a trip current using a set value resistor may create varying fault currents depending on the supply voltage. It is therefore important that the RCD tester is only used within the specified voltage range of the RCD tester. The more advanced RCD testers will create a trip current at the required value, regardless of the supply voltage. This technology is sometimes referred to as constant current circuitry.
Portable RCD's - Isolation transformers
To perform a trip time test on an RCD, a current leakage fault must be simulated. To simulate this fault, the RCD must be operating in normal conditions (ie connected to 240V AC power).
Therefore to perform trip time test on a portable RCD, the portable RCD must first be connected to 240V AC power. The RCD trip time tester is then connected to the portable RCD for the test to be carried out.
If the circuit on which the portable RCD has been connected is also protected by a fixed switch board there will be a problem...
There will essentially be two RCD's connected in series - you'll have no way of knowing which RCD will trip first during the test. Sadly it's almost always the wrong one!
An isolation transformer provides a barrier between the portable RCD and the fixed switchboard RCD. An isolation transformer is essentially no different to any other transformer, but the output is the same as the input (ie 240V AC). By using an isolation transformer, the fixed RCD cannot be tripped in error.
RCD testing Isolation transformers are configured slightly differently to some other isolation transformers available and are designed specifically for RCD testing purposes.
Help -> Looking for an isolation transformer? Click here
Calibration
Like all test and measurement equipment, RCD testers should be checked at regular intervals to make sure they are working correctly. Regular calibration and/or verification makes sure you equipment is making the correct measurements. Calibration of RCD testers should be carried out by an authorised calibration agency to a nationally traceable and recognised standard.
The cost of calibration varies according the make and model but typically calibration costs start at around $100.00. This ongoing cost must be considered at the time of purchase. Calibration intervals are normally determined by the manufacturers recommendations but would typically be annually.
For those companies operating in QLD, the electrical safety regulations state that: 'if a testing instrument can not be visually confirmed as being correctly functioning and safe—that the instrument is tested at least every 6 months to ensure it is in proper working order; and records of tests performed are kept for at least 5 years'
Careful consideration must therefore be given to what 'tested' means and what records are kept in order to meet the regulation. Many individuals including qualified electricians interpret this as a professional calibration every 6 months.
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