10.5 Constructing Safe Fixtures
As described in the previous section, when the light curtain of the safety system is broken, the tester will stop the execution of a high voltage test, and remove the dangerous voltages, to protect the operator from the dangers of electric shock.
This may sound a simple operation, but in fact it is not. Firstly, there has to be a certain amount of signal processing within the vertical columns to detect that the IR beams have been broken; secondly, the control box has to signal this to the tester; and thirdly, the tester has to ramp-down the test source. This final operation has to be done carefully, because if there is current flowing through the inductance of a winding and it is suddenly switched off, rather than creating a safe situation, it could produce a dangerously high ‘back emf’.
Clearly this means that, after the light curtain has been broken, there has to be a finite amount of time to switch off and safely ramp down the potentially dangerous voltage. With the IR Safety System, this time could be as long as 50msec.
This time has to be born in mind when you are designing your test fixtures. It is essential that any high voltage point on the transformer under test is set far enough back from the light curtain so that it cannot be touched by the operators hand within 75msec of breaking the beams.
Note that the high voltage points on the transformer could include the body or core (as well as terminals) if your program contains Hi Pot tests for core isolation.
The normally assumed value for the speed of an operator’s hand is 2m/sec. This speed implies that the distance from the beams of the light curtain to the dangerous point on the transformer should be at least 150mm.