Among electricians, the common testing topic which arises revolves around Earth Loop Impedance. Here we shall discuss what the test is about, the difficulties faced during the test, and the different methods used to carry out the test.
Earth Loop Impedance: What is it?
When a short circuit occurs in the electrical circuit board, the current flows from the line conductor towards the earth and touches the neutral point of the electrical transformer.
The circuit loop is made up of the supply transformer winding, phase conductor, main fuse and switch, line and earth conductor, protective device, main earth terminal, altogether it makes up the Earth Fault Loop. Ohm is the value in which earth impedance is measured.
When a circuit is installed on the premise, it should be ensured that all the devices in the installation are safe and well protected from receiving high voltage which burns out the electrical devices. The value of earth loop impedance is measured from the furthest point of the circuit and compared with the wiring regulations of BS 7671. After the test, if the value is higher than the regulations, then the circuit is deemed unfit for installation.
Earth loop testing is one of the most difficult tasks that has been carried out daily by electricians. Carrying out the test by itself is not the difficult part here but finding out accurate, stable, and reoccurring results each time a circuit is put to test. The test is simple yet keeping up the principles of continuity, RC testing and insulation become quite frustrating for the electrician.
When carrying out the test, one must keep in mind that what level of resistance can be the live wire and the earth be subjected to in case of a high voltage frequency. Over the years, test equipment manufacturers have been looking for a solution to find accurate and stable results for each time a circuit is installed. But due to constant changes in technologies, this becomes a hurdle for manufacturers and electricians.
Why do we need to test the loop?
In case a fault appears in the circuit, the loop must be reliable enough to keep a smooth flow of electrical current and maintain proper functioning of the circuit, in this case, the extra energy generated is directed to the earth system of the wiring component.
The value of impedance changes with the type of installation of the circuit, type of applied protection which could be a miniature circuit breaker, cartridge, or rewireable fuse. The fault can occur in the neutral line (connecting to the transformer) or the Line Earth (connecting to the earth) so it is pertinent to carry out a loop test for each.
Earth Loop impendence test methods:
Generally, an electrician follows 5 different methods to test earth loop impedance:
- 2-wire high current test method
- 2-wire ‘No-Trip’ d.c. saturation test method (Obsolete)
- 3-wire ‘No-Trip’ method
- 2-wire ‘No-Trip’ method
- 4-wire grid impedance test method
1. 2-wire high current test method:
It is one of the most widely used and traditional methods of carrying out the impedance test. In this method, the highest voltage is measured using only two wires on a circuit. It is also the fastest and most accurate test. This is mainly carried out by electricians as it enables to test at a very high frequency and can be carried out on a day to day basis. This test often returns accurate and repeatable results.
2. 2-wire ‘No-Trip’ d.c. saturation test method (Obsolete)
In this technique, a DC current is injected into the circuit board to saturate the coil monitoring in the RCD as it provided enough time for the AC test. This is carried out before the two-wire test is carried out. This technique was developed to overcome the problem of earth leakage.
3. 3-wire ‘No-Trip’ method
This no trip loop method is being used for over 20 years now. This is a low current Line Earth test and always returns accurate results.
4. 2-wire ‘No-Trip’ method
This method has changed over the years and offers far better repeatability of results but with limitations because external factors interfere it.
5. 4-wire grid impedance test method
This test is only applied in different circumstances as it uses only a 4 wire Kelvin connection by leaving out contact resistance and internal lead. This also generates one of the highly accurate test results. A measurement as low as 10mOhm can be accurately made with this type of testing.
Earth Loop Impedance Test in a nutshell:
These testing are the new challenges posed by the ever-advancing technology and if we look back at the 1970s, the measurements were far simpler, lying below 1 and 2 ohms. With newer technologies, there are new challenges and new methods and we are always striving for perfection.