Safety Testers Take on Routine but Essential Tasks

Designers, marketers, and vendors alike accept product safety as paramount. It’s the one area where we all agree that there are no compromises. Nonetheless, failures in design, production, or in everyday use do render products unsafe, and these rare occurrences are valuable reminders to never become complacent about safety.

Vendors in Europe have always paid high attention to product safety testing, but the introduction of the Low Voltage Directive (LVD) in 1997 raised the profile of electrical safety testing even further. Before the directive many T&M product vendors conducted electrical safety checks as part of an overall type approval test at product introduction. Since the directive’s introduction, test vendors now apply safety checks to 100% of production output. Vendors see 100% testing as the surest way to prove on-going conformance to the LVD and as the best way to gain peace of mind.

For T&M products, all the electrical safety tests apply to the unit’s mains input and are made via the mains input connector. The tests check a product’s protective earth, its insulation resistance, and its dielectric strength.

The aim of the protective earth test is to measure the resistance of the internal bond between a product’s earth pin on the mains connector to any accessible metal parts on the product. The test limit is less than 100 mV for the bond only, which means the tester must allow for the additional resistance of the mains cable. In practice, for a product that consumes less than 6 A the test allows up to the 28 mV/m in the power cable.

The insulation test measures the resistance between the line and neutral joined together and the earth. For T&M instruments, the tester applies 500 V DC and checks for a resistance path of more than 2 MV.

The dielectric strength test (also known as a flash test, or HIPOT test) applies 1.5 kV ACV or 2120 V DC across the same points as the insulation test. There should be no breakdown in this test and current should settle to less than 5 mA.

You can purchase individual instruments to perform these tests separately, but our product survey covers safety testers that perform these three essential tests and also a few more (see the product survey table). For an instrument in this class you should expect to pay around FFr50,000/DM15,000/£5000. With these testers, you can simply plug the supply end of the T&M product’s mains cable into the safety tester and perform all the checks via the mains cable. These testers are also programmable, so you can set the parameters for the series of safety tests you want and run a complete test automatically.

Once you have a tester connected to an instrument via its mains cable, it’s useful to do a quick functional check on the supply current with the instrument operating normally. An out-of-limit result in either direction could provide useful indication of a potential overload condition or just a blown fuse. Some of the testers in the survey also provide this feature.

Earth Protection Is Basic Test

During the earth bond test you use an additional hand probe to press onto any accessible metal on the product’s casing. The open circuit voltage for this test is below 12 V, but the test current depending on bond resistance can be up to 25 A. So, don’t be surprised if you see a few sparks. Typically, you should set the test current limit to at least 1.5 times the regular supply of the instrument you’re testing. Other testers provide a large crocodile clip that you can attach to a product if a convenient test point exists.

a)

b)

c)

Figure 1. Edgcumbe Instruments contrasts the dielectric strength applied voltage waveshape from other safety testers (a) & (b) with its own Elite 2 tester (c). Overshoot and ringing (a), and residual charge (b), are eliminated by a controlled ramp-up, 2-sec dwell, and ramp-down (c).

There are some sensitive test products — such as signal conditioning boards or modules — that have conductive cases that primarily provide EMC screening rather than earth protection. In these instances, connections to the screen may consist of the outer screen on a signal cable or a printed circuit board track. These products still have to conform to the LVD. Clearly, driving up to 25 A down these lightweight connections is not good sense and the LVD recognises that. In such cases, you can apply a lightweight continuity test with an open-circuit voltage of around 100 mV and a current limit of 100 mA.

Mains Filters Cause Hiccup

One problem you’re likely to encounter in safety tests is with mains input filters. T&M products generally have a filter network integral with the mains connector to help reduce conducted noise. Problems can arise when your dielectric strength test suddenly applies 1.5 kV ACV and the filter capacitors pass excessive current. In one case, the in-rush current can damage or degrade the capacitors. In another case, the steady-state AC current through the filter can cause a dielectric strength false fail indication.

One way around this problem is to apply the 2120 V DC dielectric strength test instead of the 1.5 kV ACV test; although it’s still possible that the initial capacitor in-rush current could damage the capacitor. The surest way to defeat the problem is to use a safety tester that can apply a ramp rather than a step voltage test (see Figure 1). In the first place, a ramp is less likely to result in overshoot or ringing that could over-stress the instrument under test. And, secondly, a controlled ramp-up followed by a controlled ramp-down ensures that the filter capacitors discharge fully and don’t deliver you an unpleasant shock if you happen to finger the mains plug after removal.

T&M Holds Good Safety Record

Wayne Harris, laboratory manager with T&M instrument hire company Microlease (Harrow, UK), says it is company policy to perform electrical safety checks on every unit that leaves for hire. The company ships out around 1000 units/ month of a wide range of instruments and Harris recalls only two instances over several years when instruments failed. On each occasion the failure was due to breakdown in an input mains filter.

As routine, Micro-lease performs an insulation resistance test on all T&M units (class I and II) together with a protective earth test on units with an earthed conductive case (class I). As regards the time the tests take, Harris says they are fast, with the most time taken up connecting and disconnecting the mains cable. As for a dielectric strength test, Harris regards that as the instrument vendors’ responsibility to be done as part of manufacture.

If you’re a typical user who operates instruments in an office/laboratory environment, there are no hard and fast rules about how often you should re-apply electrical safety checks. Based on the Microlease experience, T&M instruments’ safety records are good. Nonetheless, re-testing once a year is well worthwhile just for your peace of mind. T&ME