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Electronic Trip Units are essentially an Overcurrent Protection Relay which protects electrical circuits and they are integral to Low Voltage Circuit Breakers only. They do not come with High Voltage Circuit Breakers.


They serve essentially the same functions as the conventional, external OCEF relays that we commonly see on LT switchboards. The sole difference is that the Electronic Trip Unit is built into the Circuit Breaker (CB). 


Its characteristics are also similar to the conventional OCEF relays, i.e. they are IDMTL or DTL. They are very versatile and allows the user to choose and configure the settings easily.


Should an overcurrent or short circuit occur in the electrical circuits, the Electronic Trip Unit will sense this abnormal condition and will send a signal to open (or to switch off) the CB thereby effectively isolating the abnormal condition.


The question then is: What is the difference between the built-in Electronic Trip Unit and the external OCEF relay?


Let’s examine first the conventional external OCEF relay:

When an overcurrent occurs in a circuit, the OCEF will sense this abnormality and closes its output contact thereby tripping the CB via its shunt trip coil. The shunt trip coil is typically rated 230 Vac. Should a severe short circuit occur in the circuit, the OCEF will still do its job; however, the shunt trip coil may not operate because there may not be 230 Vac in the system anymore! A short circuit causes the voltage to drop drastically.


One way to overcome this is to have the shunt trip coil rated say 30 Vdc and have a 30 Vdc battery charger unit in the switchroom. Another way is to use the built-in Electronic Trip Unit of the CB. The trip unit does not use external power source to trip the CB. Instead, it uses the energy from the fault current to trip itself.


In general, most local users adopt the external OCEF relays for overcurrent protection coupled with the Electronic Trip Unit for short circuit protection.


These micro-processor based Electronic Trip Units are quite sensitive and delicate electronics which are subject to the harsh conditions of switchrooms. To ensure that they function when called upon, we have to carry out routine checks.


Besides the conventional current injection test set which are physically large, heavy, cumbersome and time-consuming to mobilise and set up, we are now equipped with dedicated electronic trip unit testers to carry out routine testing for different brands of LT circuit breakers more efficiently.


Information presented in this website is correct to the best of our knowledge; presents our opinion. Viewer discretion is advised.

STEPL and its employees shall not be liable for any inaccuracies or for any special, incidental, direct or indirect consequential damages, loss of profits, production or business interruption arising out of and in connection with the materials provided on this website.

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