It’s funny how the same issues come round regularly. The first time I saw these was in the mid 90’s when I was shown a family of power supplies that had been built into the sort of plastic boxes you can get from Radio Shack.

Now, there is nothing wrong with plastic boxes from Radio Shack – until you try to use them in an application for which they never intended – for example Building Power Supplies.

The client needed a number of “Cheap” power supplies to power instrumentation that connected to a computer – and these power supplies were certainly cheap. . . . .

These PSU’s also needed to comply with EU Safety Directives (it was before the days of REACH and RoHS, which now apply to most products.).

Plastic was to be the material of choice for the boxes (..”because it is an insulator”…).

A simple “E-I” core transformer took the incoming 230V AC to about 15 Volts AC at about 100 mA and provided Basic Insulation across the bobbin between Primary and Secondary and between Primary to the transformer core.

Capture Capture2The designer had applied “good engineering practice” and added an internal 20 mm glass fuse rated 1 Amp (T) 250V.

All fastenings for these items were steel with ‘nylock’ nuts.

 

Let’s forget about applying any specific standard to this design and let’s just list a few of the obvious potential hazards. You should need only the descriptions given to identify the potential hazards and their solutions.

Electric Shock – (a) from the screws fixing the transformer core and (b) screw holding the fuse holder; (c) from a failure of Basic Insulation between Primary and Secondary winding to dc output

Fire – (d) the plastic had a UL94-HB rating ( which limited its use to a decorative enclosure);
(e) the fuse rating would not protect the transformer from fire; (f) a glass fuse will have a typical maximum “breaking” current of only about 30 Amps. This means that a Live Neutral fault could create a fault current in excess of 2,000 Amps – causing the fuse to explode and create a plasma that would sustain the fault current – perhaps for several seconds.

Mechanical – (g) the box cracked during the steel ball test and allowed a test finger to contact live parts; (h) the box failed a drop test – the transformer fastenings pulled through the box and rattled about inside the box smashing everything inside; (i) the external (slotted screw) fastenings could be turned by the user and internal insulation barriers could be bridged.

QUESTION – with only the information given above:  Which hazards cannot be solved if the box is an electrical insulator? –   If you select a better grade of plastic for the box, how would you solve the remaining non-compliances?

 

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