The phase rotation test is a common test carried out by electrical installers as a safety precaution when the order in which loads have been connected to the phases has been altered, or to check that the load connections are connected to the correct phases.
What is phase rotation?
Electricity is mainly delivered in a three phase system, and the phase rotation – or phase sequence – of this system has to be correct. Phase rotation is the order in which the voltage waveforms of a three phase system reach their peak values. In a three phase system there are three separate voltage waveforms which are 120 degrees out of phase with each other. The order in which these waveforms reach their maximum value determines the phase rotation of the system. Ideally the waveforms are balanced and each remains equally spaced 120 degrees apart.
In practical terms, phase rotation is important because it affects the direction of rotation of items such as three phase motors, and the performance of other electrical devices that work in rotation, such as transformers, generators and inverters. Single phase loads are distributed evenly and correct phase rotation ensures the phases aren’t overloaded if other phases have plenty of available energy. If the phase rotation is incorrect this can lead to the inefficient operation of the item in question. This can then lead to increased energy consumption and damage to the device.
Another important function of phase rotation is that it ensures the proper operation of safety devices, such as those that protect short circuits, Earth faults or overloads on a system. For these safety devices to operate properly, and to ensure the safety of the operation and system, the phase rotation has to be correct.
Carrying out a phase rotation test with a multi-function tester
Going back to the above explanation of phase rotation, the phase rotation test can detect the sequence of voltage waveforms present in a system and indicate whether the system is rotating in the correct direction. This is a very simple test and is commonly carried out using a multi-function tester such as the award-winning MFT-Pro supplied by Test Instrument Solutions, which also has a single pole Phase Rotation method for further ease of use.
The manner in which a multi-function tester indicates the correct phase rotation will differ from one instrument to the next. Some use an LED light, but most modern instruments such as the MFT-Pro indicate the result on an LCD display screen.
A phase rotation test can be carried out at the origin of supply and at each subsequent distribution board in the system.
On your instrument you want to run the L1, L2 and L3 sequence, so:
- Connect the three leads to the three phases in the distribution board
- Select the phase rotation test on the screen (or turn the dial manually to the phase rotation test if you are using a different instrument)
- You should see a reading on the screen indicating L1, L2 and L3
- If these results are shown in the right order then the sequence is correct and rotation is the right way round
- You can switch the cables around and input them in different phases and run the test again. This should show a different sequence, such as L3, L1 and L2, or however you have inputted the leads, and is a check that the instrument is operating correctly and your understanding of the installation is also correct. As well as showing the incorrect sequence, the numbers on the display may also flash as a more visual indication that there is a problem.
Please note that this section is for information purposes only. Anyone using equipment referred to in this section must be suitably qualified and/or experienced within the respective field. If in doubt before use, please consult a qualified electrician or engineer & thoroughly read all instruction booklets.