|Checking Phase Sequence|
| 08 Sep 2011
One of the requirements of BS 7671:2008 incorporating Amendment 1:2011in Part 6, Regulation 612.12 is to verify that the phase sequence in multiphase (ie. three-phase) circuits is maintained.
But what does this really mean? This article will explain what phase sequence testing means and the correct procedure.
What phase sequence means
Correct phase sequence means that the phase sequence of the incoming line conductors (ie. Brown-L1, Black –L2, Grey-L3) is the same throughout the installation.
Phase sequence testing is testing to confirm that this is the case. This is especially important for large industrial installations, which have three- phase motors which are required to rotate in a specific direction.
It is not uncommon in large industrial installations, which have a number of sub-distribution boards, to find that the incoming line conductors at each distribution board are connected in a different sequence to that at the incoming three-phase supply to the building.
An example of an installation where the correct phase sequence has NOT been maintained is illustrated in Fig 1. Incorrect phase sequence on existing installations is commonly found in installations which have been subjected to additions or alterations over the years involving the installation of new three -phase motorised equipment, which, when it is commissioned, is found to rotate in the wrong direction.
It is sometimes incorrectly assumed that a phase rotation test instrument will verify the phase sequence of a multiphase circuit.
Phase rotation verification confirms the phase rotation at a particular item of equipment but it does not verify that the phase rotation at that item of equipment is the same as the phase rotation at the incoming supply.
The test instrument will give an indication that the three phases are separated by 20 degrees (see Fig. 2) and, this being the case, it will give a correct indication whether the test leads are connected Brown L1, Black L2, Grey L3 (as in Fig. 3) or Grey L1, Brown L2, Black L3 (as in Fig. 4).
Therefore, it must be emphasised that the phase rotation tester cannot verify that the phase sequence of the incoming supply has been maintained throughout the installation.
The correct procedure
The proper method of confirming that the phase sequence is correct is to isolate the installation from the supply and carry out continuity testing of the individual conductors from the origin of the installation to each distribution board and from there to the furthest point of each circuit. This testing may need to be carried out in sections.
Once the supply polarity including phase rotation has been confirmed for each section, the power can be restored and the next section tested until the phase sequence has been confirmed throughout the installation. Phase sequence testing can then be carried out at each item of equipment.
Checking the phase sequence of three-phase equipment
As each item of equipment is energised, it can be tested to confirm that its phase sequence is correct using a phase sequence/rotation instrument.
The test instrument will give an indication that the phase sequence is correct. by confirming that Brown is connected to phase L1, Black is connected to phase L2 and Grey is connected to phase L3. Figures 5 shows an correct connection and figure 6 shows the incorrect method.
With three-phase circuits, problems arise where items of equipment, normally motors, are found during the commissioning stages, to be operating in the wrong direction. The correct method of rectifying this problem is to alter the equipment’s internal wiring at the terminal block. The manufacturer’s instructions should be consulted prior to undertaking this task.
Unfortunately, the method often employed to correct this fault is to change the connections at the local isolator or at the distribution board supplying the equipment.
This then creates a situation where the phase sequence of the multiphase circuit is not maintained and therefore is in contravention of Regulation 612.12.
Testing the phase sequence of single-phase circuits
Although Regulation 612.12 refers to multiphase circuits, it is important in a multiphase installation which supplies various single- phase circuits that the phases L1, L2, & L3 are confirmed as part of the verification procedure.
This is to confirm that any single-phase circuits of the installation that are in close proximity to each other or in common enclosures are identified as to the correct line conductor. Any necessary warning or identification labels can then be provided.
Phase sequence testing is an essential test and it is of the upmost importance that electricians understand the correct procedure and have the correct test instrument.