Lister Petter Generators

Lister Petter diesel engines and Lister Petter diesel generators are world renown for their reliability.We've seen how you can connect electrical appliances in parallel, but what happens if you connect cables in parallel? In other words, rather than running one pair of wires to each appliance, why not run two? How would this help? Well, if there are two sets of conductors running to each appliance, this is exactly the same as having one set of conductors but with twice as much copper area. And a conductor with twice the area can carry twice the current (for reasons I'll explain later).

So if we double the number of cables connecting each appliance, we double the amount of current they can carry. Another way of looking at this is to say that if we double the number of cables, they only need to have half the area, and thin cable is cheaper than thick cable.

This principle is exploited in the wiring of `ring' circuits in domestic installations. Rings are almost always used in wiring power outlets, and sometime in lighting as well. In a ring, every socket outlet has not just one live, neutral, and earth connection back to the supply, but two; this is because the ring goes all around the area served and then back to the supply.

This also explains why it is so dangerous to allow a ring to become broken. In this situation there will only be one set of conductors serving each power outlet. Some outlets will be on one side of the break, and some will be on the other. So all will get a supply, and it isn't obvious that anything is wrong. However, a double-gang 13-amp socket can draw a current of 26 amps if two heavy-duty appliances are plugged in, and this may well be too high for a single run of 2.5 mm² cable, but well within the capabilities of two such cables. There is a very real risk of the cable overheating.

In normal circumstances it is impossible to plug in enough appliances to damage the cabling. Why? Because the fuse or MCB has been chosen to suit the current rating of the cable (see below). In a ring system, we will choose the fuse or MCB to suit the capacity of the ring, not a single cable. The fuse will normally be rated to trip at about 30A, which is well within the capacity of the ring, but close to, or above, the capacity of the single cable. So the fuse won't protect us from plugging in two 13-amp appliances: 26 amps isn't enough to trip the fuse, and the cable will overheat instead.

In domestic electrical work, current is generally measured in amps. Currents you will encounter in practice range from about 0.5 amps (through a lightbulb) to about 40 amps (an electric shower). Technically `amps' is short for `Ampиres', but the full name is now rarely used. The mathematical symbol for current, as it is written in calculations, is not `C' (for current) or `A' (for amps) but in fact `I'. This is just because the symbols `C' and `A' are reserved for other things. You will occasionally come across currents measured in milliamps (`mA' for short). A milliamp is a thousandth of an amp. For example, most earth-leakage breakers used in domestic wiring trip at 30 mA, which is about one thirtieth of an amp.

So all will get a supply, and it isn't obvious that anything is wrong. However, a double-gang 13-amp socket can draw a current of 26 amps if two heavy-duty appliances are plugged in, and this may well be too high for a single run of 2.5 mm² cable, but well within the capabilities of two such cables.

There is a very real risk of the cable overheating. In normal circumstances it is impossible to plug in enough appliances to damage the cabling. Why? Because the fuse or MCB has been chosen to suit the current rating of the cable (see below). In a ring system, we will choose the fuse or MCB to suit the capacity of the ring, not a single cable. The fuse will normally be rated to trip at about 30A, which is well within the capacity of the ring, but close to, or above, the capacity of the single cable