External Earth Loop Impedence Test
During any electrical works an external earth loop impedence test may be carried out to obtain the value of Ze
What is an earth loop impedence test?
An earth loop impedance test, often simply called a loop test, is performed to verify that if a fault occurs in an electrical installation, sufficient current will flow to operate the fuse or circuit breaker protecting the faulty circuit within a predetermined time. The objective is to make sure that the circuit is disconnected fast enough to prevent overheating and possibly a fire
If a “fault current” is left undetected, circuits often overheat and can even cause a fire
The test involves applying a known voltage and measuring the resulting current flow in the circuit. The ratio of voltage to current gives the impedance value in ohms.
There are different methods for performing an earth loop impedance test, depending on whether the circuit is protected by an RCD or not. For circuits without RCDs, a high-current test can be used, which involves connecting a tester between the live and earth terminals and injecting a current of 20 A or more.
This method is fast and accurate, but it will trip any RCDs or RCBOs in the circuit. For circuits with RCDs, a no-trip test can be used, which involves connecting a tester between the live, neutral and earth terminals and injecting a current of 15 mA or less.
This method does not trip any protective devices, but it may be affected by external influences such as noise or parallel circuits.
The test results should be compared with the maximum allowable values given in BS 7671 for different types of protective devices and circuit lengths. If the measured impedance is too high, it means that the fault current will not be enough to operate the protective device within the required time, and the circuit may be unsafe.
Why are my earth loop impedence readings too high?
One of the common questions that electricians face is why their earth loop impedance reading is too high. Earth loop impedance is a measure of the resistance of the circuit path from the main earthing terminal of an installation to the earth electrode of the supply system and back to the source transformer. A high earth loop impedance reading can indicate a fault in the earthing system, such as a loose connection, corrosion, or damage. A high earth loop impedance reading can also affect the operation of protective devices, such as circuit breakers and residual current devices, which rely on a low impedance path to earth to operate correctly. Therefore, it is important to identify and rectify the cause of a high earth loop impedance reading as soon as possible.
In the case of a high earth loop impedence reading at your electrical supply it is advised to contact your supply authority and they will investigate the problem.
What is the danger of high earth loop impedence?
High earth loop impedence can cause several problems for an electrical installation, such as:
- Reduced fault current that may not be enough to trip the circuit breaker or fuse in time, leading to overheating and fire hazards.
- Increased voltage drop that may affect the performance and lifespan of electrical equipment.
- Increased risk of electric shock for people and animals that may come in contact with exposed metal parts.
To prevent high earth loop impedance, it is important to have a proper earthing system that provides a low-resistance connection between the electrical installation and the earth. The earthing system should be regularly tested and maintained by a qualified electrician.
Can an earth loop impedence reading be too low?
It is important to ensure that the earth loop impedance is low enough to allow sufficient current to flow in case of a fault, so that the protective device can operate quickly and safely. However, there is no definitive answer to whether an earth loop impedance reading can be too low, as it depends on the type of system and the value declared by the electricity distributor. For example, for a TN-S system, the typical maximum value of external earth loop impedance (Ze) is 0.8 Ω, while for a TN-C-S system it is 0.35 Ω. Therefore, a reading lower than these values may not necessarily indicate a problem, but rather a good quality of earthing. However, if the reading is significantly lower than expected, it may be advisable to check for possible sources of error, such as incorrect connections, faulty equipment or parallel paths.
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