BANG!

Photos or it didn't happen!

It was my favourite screwdriver too.

George B.

I have built a lot of hand-laid N-gauge model railway track using copper-clad sleepers. The sleepers are all gapped to prevent shorts but sometimes there are stray bits of copper bridging the gap which creates a big headache when fault-finding. I had an old Triang controller with 15V AC output rated for 2A with a relatively slow-acting thermal overload. I used to hook that up to each section of track and turn it on. Any stray strands of copper didn't last long.

Due to crass stupidity on my part (I'll say it before someone else does...) I managed to put a bolt through the mains feed into my CNC control box when reassembling after maintenance recently. It took out the 32A ring main mcb plus upstream rcd in the consumer unit but two 13A fuses in plugs were untouched. So, if you want to avoid blowing fuses, just fit an MCB...

This episode actually shows how poor fuses can be for protection, and how people who talk about "protecting equipment by fitting a fuse" are often deluding themselves - fuses, and better still MCBs, are to stop the wiring bursting into flames after the equipment had already failed catastrophically!

Nealeb, fuses are probably the safest of all the circuit protection devices as they are fail-safe as long as they have not been interfered with. If they are from a reliable supplier they will fail according to a standard.

MCB's on the other hand may or may not always work to the standard to which they were produced for a number of reasons. Age and environment (temperature and damp for example) being the main cause of unpredictability. If an MCB rated at 3K breaking capacity has been exposed to a dead short which has produced a greater fault current than which it was designed to then it is likely to be permanently damaged and will probably not disconnect correctly, if at all.

RCD's and RCBO's have a high failure rate which is why they have a test button on them - it is not for show and should regularly be tested as it may offer no protection from stray residual currents. We have regularly found RCD's and RCBO's that have failed out of the box.

So the statement " shows how poor fuses can be for protection" is totally incorrect. MCB's are not as reliable as fuses which is why they are still used in such numbers in industrial situations.

You are right, however, when you say that the fuse/mcb is there purely to protect the installation (wires).

Edited By Steve Skelton 1 on 24/11/2020 12:54:08

At the risk of leading this thread astray for a moment, Robert Atkinson2 said yesterday here that all VFD's should be in enclosures. I quite agree. I have two I would like to out into enclosures but can't find any the right size. Before I make some out of perhaps plywood with perspex front, does anyone know of a good source of enclosures for VFD's of all sizes?

Chris

Chris

CPC list almost 2000 different enclosures on their website. **LINK**

Stuart

During a previous life as an x-ray service engineer I was tasked with changing a high voltage "tank" in a hospital x-ray room that had developed an internal switching fault. (This was basically a large steel tank filled with oil and containing a high voltage transformer and switch gear.) The installation was ancient (valves and relays etc) and consequently the company had to source from somewhere in the Balkans.

I followed the standard procedures for setting the correct waveforms for the tube output and had set the oscilloscope to trigger on the final test at maximum current and voltage. As I pressed the exposure switch there was an incredible bang and a blinding flash from the control console that I was kneeling behind. The full output of 30kW @125kV had shorted inside what turned out to be a faulty tank with the result that part of the control cabinet was damaged.

My eyesight returned to normal quite quickly and the ringing in my ears subsided, but my bowel problems persisted for some time!

Gordon

I bought an enclosure from TLC for my mill VFD, the panel was supplied with the door hinged on the right with the gland plate on the bottom, the door is swappable to get the gland plate and your preferred hinge side. I have a local branch so easy to collect. They have a website so you can peruse the sizes and prices. TLC Electrical

Mike

Thanks for the enclosures links lads - i'm checking them out

Chris

A removable backplane is most desirable and not always included, most of us are pressed for space but specifying an enclosure that is as small as possible can be difficult to work on. A removable backplane means the inter component wiring and the fixing of the components can be done out of the box. Consider how to replace components without having to strip the whole backplane out, tapping the backplane rather than using nuts and bolts will help. Nut inserts can also be useful for fixing to the backplane, plastic slotted trunking can keep things tidy and also consider using top hat rail for component and terminal mounting, many control devices will have the option for rail or direct mounting.

Mike