Feb 3, 2019

The proper MCB ?


EARTfelt Welcome dear friends of protection and control engineering. In our new two-part article, we show you what is really important in the design of miniature circuit breakers (MCB's) for voltage transformer circuits. In today's first part, we give you a design guide to help you choose the right size.


We all know that a voltage transformer is a "voltage source" which must be protected with a fuse against short circuit, and thus also against thermal overload. Motor protection switches are usually used for this, but miniature circuit breakers with low rated currents can also be used. In the field of low voltage  VT's are hardly used for the realization of protective functions. Therefore you can not do much wrong with these systems. However, as soon as protective functions with voltage-dependent criterion are used, the protection of the voltage transformers is of great importance. This applies in a special way for the distance protection. Since the distance protection determines a false impedance in case of failure of the measuring voltage, which can partly result in an instantaneous tripping, special caution is required here. To safely block the protective function, the auxiliary contacts of the circuit breaker are traditionally used. If there is a short circuit in the secondary circuit, the MCB should clear the fault via its fast electromagnetic release and report the trip via the auxiliary contact. This blocks distance protection or other protective functions with voltage criteria.


Important when selecting the voltage transformer circuit breaker is the tripping current. At the same time, even far-away faults should lead to safe triggering. Therefore, the cable length up to the protection device is of crucial importance. With 110 kV substations with central protection rooms in the switch house or in power plants and industrial plants, large distances and thus high line resistance can be achieved very quickly. As with any other system, the loop impedance is decisive for the magnitude of the short-circuit current. In normal low-voltage installations, as a practitioner, one would simply use an on-site meter to determine the impedance of the loop and the short-circuit current. For circuit breakers in the secondary circuits of voltage transformers this is not a practical solution. On the one hand, the measurement here is not quite so trivial and on the other hand it is important not to first determine during commissioning that the circuit breaker used is unsuitable. The planner of a new system must therefore determine the minimum secondary short-circuit current and select on the basis of a suitable MCB.

Minimum short-circuit current

With the aid of a calculation program, the approximate short-circuit current can be estimated. Unfortunately only rough, as the manufacturer of the VT makes in most cases, no indication of the minimum short-circuit voltage of the VT. The minimum short-circuit current is also mainly influenced by the line resistance. In practice you got a chance to measure the short circuit power as you would do it with a big transformer: You prepare a short circuit of the low voltage side and feed a HV-current until the rated current on the low voltage side is reached. Now you measure the voltage, it's the short circuit voltage which you can use for the following calculation.

In the table below we have compiled an overview of typical high-voltage transformers with copper wiring circuit. The table shows the initial short-circuit current Ik1p" as a function of the cable length and the cable cross-section.

Which conductor cross sections are used is often determined by specifications of the operator. From our personal experience, we are familiar with specifications in the range from 1.5 to 6.0 mm². In order to configure a safe design of the MCB, the required current for a short-time trip should not exceed 50 % of the determined minimum short-circuit current.


The grid operator of a 110 kV substation has specified a 2.5 mm² NYY copper wire conductor cross section. There are about 70 m between the new control field and the existing substation building. Due to the cable routing, the cable length is about 100 m. According to our calculation above, a fault directly on the protection device will cause a minimum short-circuit current of approx. 34 A. Since the short-circuit current is further reduced by contact resistances at non-optimal terminal points and above all by the fault resistance, we halve this value again and obtain 17 A.

The manufacturer Siemens has three special circuit breakers for voltage transformers on offer. Among other things, the data sheet identifies the response value of the instantaneous electromagnetic overcurrent release.

You can choose from:

🌐 6 A ± 20%

🌐 10.5 A ± 20%

🌐 20 A ± 20%.

The circuit breaker with 10.5 A ± 20 % would be the right choice for our system. At the value of 17 A calculated by us, this would trigger within approximately 8 ms. The circuit breaker is also designed for a rated current of 2.5 A.

In case of doubt, we recommend you always to use the smallest circuit breaker. There is no great risk here because the secondary operating current of the voltage transformer is, in most cases, only a few milliamps. Typical exceptions are meter circuits with Ferraris counters, analogue instruments and synchroscopes. Here you have to take a closer look. We have to mention as always: You got to test, to test and to test again. You always should practice a real short circuit test in the end of the secondary circuit to see it the MCB is working properly.

In the second part of our article, we are dedicated to an intensive consideration regarding the auxiliary contacts of the MCB's. Again, there are some important things to consider.

HEARTfelt Greetings Alex