What they do
The design of the DOL starter is such that it encompasses vital safety features in order to comply with the requirement of BS7671 that machines are not capable of unexpected starts that may cause danger. It does this through the contactor which is held in the 'closed' position only when there is current passing through a coil that controls the position of the contactor i.e. open or closed. This coil is called the 'low or no-volt coil'.
What this means is when the supply to the DOL starter fails then the contactor is opened (off) and if the supply is re-instated the only way of re-energising the motor or other equipment is by pressing the green 'start' button again (on). Obviously all DOL starters include for safety an emergency stop button.
Whats the problem?
So why do we care about it and what does it mean during inspection and testing? There are two reasons to take DOL starters into account; testing of them is slightly more complex and they must undergo a functional test.
Continuity testing circuits supplying DOL starters
As we have discussed the DOL circuit has an input circuit and an output circuit. These are joined by the contactors (think of them as an electrical switch) between them. If the contactors were replaced by a normal switch then simply closing it would give us a continuous circuit from source of supply to the point of use. This would enable us to measure R1(L1)+R2, R1(L2)+R2 and R1(L3)+R2 to complete our entries on the schedule of test results.
However in order to do this on a DOL starter with a contactor the circuit would have to be live as the contactors are closed by a live supply. Obviously this is not an option. Many folk by-pass the contactors by connecting short lengths of conductor between the input and output terminals of the contactor. It works, but it is a fiddle and you risk a poor connection when you remove the 'jumpers'. This may be a convenient way of performing continuity tests if the circuit is very long or the use of an R2 wander lead is not possible.
An alternative and valid way of performing this is to think of the R1s (L1, L2 and L3) in sections to be added together. So for instance measuring the continuity of L1 from the CB to the input connection of the contactor and then adding that to the continuity of L1 from the output connection on the contactor to the motor terminal or socket. R1 (input) + R1 (ouput) = R1. Do this for all 3 phases and then add to each one the R2 (which of course will be continuous end to end) and you will have an R1+R2 for each phase.
Of course if there is any great distance between points of measurement then you will have to use a nulled R2 wander lead if safe and convenient.
Insulation Resistance testing
Continuity testing is not the only test where a DOL starter's split circuit causes us problems. If you recall the corrent method of carrying out an IR test is to test at the output side of the board's main switch, testing all circuits and bonding including exposed and extraneous conductive parts at the same time. Thats all well and good but you will not have tested all parts of the system as required by BS7671, namely the output circuit from the DOL starter to the end point.
There are 3 ways of performing this. You could just manually close the contactor (if you have a monkey or a spare hand), you could jump the input/output on the contactor or simply insert your probes into the output terminals of the contactor in turn and test against the other Lines or cpc as required, recording the lowest reading in all rows of 'line to line' 'line to cpc' columns for that board.
Finally a simple functional test is required. Simple it may be but it must be done correctly. With the board energised and the breaker for the DOL starter circuit 'on' the 'start' button on the DOL starter is pressed. You should hear a click as the contactors are closed (a phase rotation tester plugged in can also indicate that the circuit is now live). The breaker is now switched off and another click is heard as the contactors disconnect. Close the breaker again and no click should be heard indicating that the contactors did not automatically close again (safe, no unexpected restart). Pressing the green 'start' button should cause the contactors to close with a click (indicating a re-start under operator control) and finally pressing the red 'stop' button should cause a click as the contactors open once more.