Earth Leakage Measurement Methods at High Current
Monitoring Device Product Engineer
Leakage current is almost everywhere the electrical system is used. When you are blindfolded, it is a subject current leakage that causes work accidents, loss of life and property. Observation and precautions taking are also mandatory by law.
Firstly, what is the leakage current, it is necessary to describe it. If the vectored sum of the phase currents and the neutral return current is zero, then there is no leakage current. But in this cycle, if the current from the neutral is less than the total current from the excess, then the electric current has found a shorter path to itself. This path can be a person directly, a point to be isolated, or any other hardware. This problem can be experienced in electrical devices which are generally insufficient insulation, manufacturing defects or outdated electrical devices.
When these problematic devices are used in systems such as electric panels, they will cause problems in the surrounding equipment. It is necessary to measure and detect before taking precautions. It is important to stop when it happen immediately. This measurement and cutting operation can be performed with direct connected devices at low current levels. Directly connected devices without setting feature set 30mA for people and 300mA for fire prevention. We can see almost everywhere these devices are used as we work.
There are situations where direct connection cannot be made in high current measurements. The current transformers that we use when installing the energy meters or analyzers are used here. Especially toroidal current transformers are the most used equipment. The current transformer usage logic is based on passing each current through the current transformer in each line to be measured and taking measurements by the secondary in accordance with the winding ratio.
In the case of leakage current measurement, the situation changes slightly. Leakage current measurement is done to protect the systems. In order to be able to measure the current difference we have explained at the beginning of the article, the vectored measurement of the sum of the total input currents and the neutrally rotating currents is required. For this reason, all phase and neutral cables are passed through these toroidal current transformers specifically together. Passed through the toroidal current transformer of course vary according to the size of the cable thickness. Openable types provides easiness at system maintenance and transformer malfunctions. The secondary side of the transformer is now ready for measurement.
The leakage current must be measured and checked after the current value is measured by the transformer. Leakage current relays are used for this task. These relays are usually manufactured as adjustable time to failure. The level of the leakage current which cannot be prevented according to the usage place of the system to be protected can be higher than the levels I just mentioned (30mA and 300mA). It is an ideal solution for users to adjust the leakage current level in such situations. This limit can be increased up to 250A current levels.
Toroidal current transformers are also available in one piece with the relay. This structure provides easiness at electric panel design and installation. The relay output directly controls the circuit breaker (such as the contactor). Thus, system protection is provided.
Due to its structure, thick cable or bare system is used in the panel which passes high current. It is sometimes not possible to pass such conductors through toroidal current transformers. For this reason, collector leakage current transformers can be used. In this system, the lines are measured by X / 5 current transformers and these current transformers are input to the collector leakage current transformer.
In the official newspaper publication dated July 17, 2013, it was stated that “the workplace’s main board and secondary electrical panels will be equipped with a leakage current relay (residual flux key) in accordance with the principle of electivity”. Accordingly, it is important to use the right solution at the right spot.