Keith Wilson - Electrical engineer
Every electrical engineer and technician is familiar with the idea of three-phase supplies, but there’s still plenty of room for confusion. To begin with, the way colours, letters and numbers are used to identify each of the phases varies tremendously around the world and, even in an individual country, the conventions may very well have changed over time.
In the UK, for example, the colours red, white and blue were used until the 1950s, when red, yellow and blue became the accepted standard. In 2006, this changed again to comply with the European standards defined in BS7671:2001 (Amendment 2, 2004). The colours now used are brown for the leading phase, followed by black and then grey.
Lettering and numbering standards are equally diverse. In Scandinavia, for example, the letters R, S, T are used, while Germany opts for U, V, W and the USA for X, Y, Z. Probably the most logical convention is to adopt L1, L2 and L3, or even just 1, 2 and 3 for the phases in order.
It’s usually thought that the normal direction of electric phase rotation is clockwise. This is, however, incorrect as the normal direction of rotation is invariably anti-clockwise. The reason for this confusion is probably associated with oldfashioned rotating-disc phase rotation testers that had an arrow on the disc pointing in the clockwise direction. It was natural for users to assume that this arrow showed the way the phases rotated but, as we have seen, this assumption is incorrect.
To get round this issue, modern phase rotation testers often show left- or right-facing arrows to indicate the direction in which the phases are rotating.
When using these instruments, beware of thinking that the right-facing arrow indicates the correct direction of rotation. It still means clockwise, and that is NOT the normal direction! Another vital point to remember is that a phase rotation meter shows which way the phases are rotating, but not which phase is which. This is an important factor that must be taken into account, so that crossed phases are avoided, every time a distribution network is updated with additional cable runs, and when cable jointing takes place.
Phase rotation testers display their results in many different ways. We have already mentioned rotating discs, although these are now obsolete, and digitally generated left- and right-pointing arrows. Other alternatives include digitally generated letters or numbers, and neon or LED indicators.
Some manufacturers use a clockwise or anticlockwise facing arrow, and many add three independent indicators to show that all three phases are present – an important requirement prior to the connection of equipment to the supply.
Possibly the easiest way to unambiguously show correct or incorrect rotation on digital instruments is to have a display that shows L1, L2, L3 (or simply 1, 2, 3) to confirm correct anticlockwise rotation, and to emphasise that this is the normal rotation by arranging for the display to be steady. For phases rotating clockwise, such an instrument would show L1, L3, L2 and the display would flash to highlight the fact that this is not the normal direction of rotation.
Phase rotation testers are an essential tool in the armoury of every engineer and technician who works on three-phase systems. Though they are often thought of as relatively simple devices, as we’ve seen there is considerable confusion over the interpretation of the results they provide. Hopefully, this short note has made things a little clearer!