Domestic and industrial equipment operating from the 230V or 400V AC power mains needs to have its insulation checked regularly, so that users can be assured that it doesn’t pose a shock hazard. After all, exposure to voltages of this magnitude can be fatal!
But what sort of test gear do you need to carry out this type of safety check? You’ll get a fair idea by reading the text in the Insulation Testing panel on the opposite page.
In a nutshell, you need a portable and isolated meter that is capable of providing a nominal test voltage of 500V or 1000V DC and able to measure leakage current or insulation resistance or both.
Our new Megohm and Leakage Current meter design is intended to meet these requirements. It is compact, portable and isolated and provides a choice of either 500V or 1000V DC as the test voltage.
It also allows you to measure insulation resistances from below 1MΩ up to virtually 999MΩ, as well as leakage currents from below 1μA to over 100μA (103μA, to be precise).
We should point out that because it can only measure leakage currents up to 103μA, it will indicate that Class I equipment (with earthed external metalwork) is effectively unsafe if it has a leakage current of more than 100μA – even though, strictly speaking, this kind of equipment is still regarded as ‘safe’ providing its leakage current is below 5mA.
So the test performed by this meter is more rigorous than the official safety standards – but where safety is involved it’s better to be too tough than not tough enough, surely?
The new meter is easy to build, with most of the major components mounted on a small PC board. This fits inside a compact UB1 size jiffy box, along with a small power transformer used in the test voltage generation circuit and the 4-AA battery holder used to supply the meter’s power. It can be built up in a couple of hours and for a much lower outlay than commercially available megohm meters.
How it works
The block diagram, Fig.1, shows what is inside the new meter. It’s split into two distinct sections: that on the left-hand side generates the test voltage of 500V or 1000V, while the metering section on the right-hand side is used to measure any leakage current which flows between the test terminals and from this calculate the external resistance connected between them.
In more detail, the test voltage generation section has a DC-AC inverter which converts 6V DC from the battery into AC, so it can be stepped up to a few hundred volts AC. This is fed to a voltage-multiplying rectifier circuit to produce the 500V or 1000V DC test voltage.