Is 50V correct or a typo? Seems suspiciously low, though my understanding of 80s Aussie telephony is not the best!

Exchanges operate at -48v DC. Makes battery backup easier.

-48V and $stupid Amps *nods*

50v is certainly the UK standard for supply to telephones so I'd imagine it's the same in Aus.

Telephony generally runs on a nominal 48 volts DC, supplied by banks of very open-vented lead-acid batteries; which have the advantage of not needing much in the way of over-charging protection, as long as someone can go round every so often and top them up with de-mineralised water. (Also, lead is pretty much indefinitely recyclable, and there's no way for it to get into your system down the phone lines.) This is the kind of power source that just melts its way through short circuits without even flinching.

If you stick an AVO across a UK phone line*, you'll get about 50 volts (with plenty of resistance in series; that would originally have been the coil of the doll's-eye in a manual exchange, later the line relay). Maybe a bit less in rural areas a long way from the exchange. There's no reason to suppose the Australian GPO would have done things any differently than at home.

* Don't hold the probes on with your bare hands as a call is coming in .....

* Don't hold the probes on with your bare hands as a call is coming in .....

Or try to strip the wires in the macho way with your teeth as a call is coming in (BTDTGTT)

48V DC is the nominal battery voltage. Actual battery voltage for typical 48V wet lead-acid batteries is 55V (higher when they are on charge). Regulation then brings that back down to ~ whatever telcos use.

I was sort-of bracing myself to some deadly spider or snake hidden in the bowels of the telephone exchange causing trouble, but no assistance from the local fauna was needed for this mishap.

It does remind me of the time I was installing a new, far more robust dimmer system in a cafe I used to frequent. Before attaching the new dimmer system to the mains supply, something prompted me to check the wiring. My suspicions were quickly confirmed when (amongst various other horrors, I found a green/yellow (earth) cable actually being used as the live 230V 16 A wire. Not something I would recommend. I corrected some of the worst transgressions before hooking up my dimmers. I also told the owner to get an electrician to sort out the other horrors I found. I am not sure if he did, but my dimmers worked flawlessly for several years, until the cafe was sold and completely renovated.

My old house had this. We had a sparky in to trace and fix some odd electrical glitches (lights in the living room dimmed when the oven was on etc) - he found that the house had been rewired over the years by (presumably) the previous owner who just used whichever electrical wire he happened to have handy; it was all sorts. Red, black, green/yellow, blue, brown, indiscriminately and indeterminately live.

There was also a very odd construction whereby the whole back of the house wasn't wired through the walls, but by a big old wiring loom run direct from the meter cupboard, laid along the attic floor and down again at the back of the house. He found this out when he was installing a new attic trapdoor for us, was using a reciprocating saw to cut the hole and managed to saw through said wiring loom which was lying on the plasterboard.

Red, black, green/yellow, blue, brown, indiscriminately and indeterminately live.

I was once helping a friend diagnose a problem with a light in her old French house. We eventually realised that the live red wire that exited from a wall was the same circuit as the green/yellow one on the other side of the wall. After digging out some plaster I found that the two had simply been twisted together and wrapped with sellotape, before being embedded in the plaster.

The house I am in now was previously owned by a guy who owned a local factory of some kind.

According to my neighbours, he had a handyman (one of his "engineers" from his factory, apparently).

Said handyman did an awful lot of electrical work.

And earthed absolutely none of it.

A quick list from memory (and there was much more): a 32A commando socket in the garage: earth bent out of the way at 180 degrees and no sheath; extractor fan in the kitchen (all stainless steel - nothing on the earth tabs from the motor feed), outbuildings (no earth at the fuseboard in the first building), three out of 5 sockets in the kitchen...no earth attached, no sheath.

Quick (wago) connectors used to join cables but then not put into the outer boxes, and left laying on a shelf in the outbuilding above head height.

Electric gate controller - nice, wire armoured cable down to it from the garage. Ah... someone did something properly? Nope... No earth tab in the compression gland. No earth.

Oh and one of the en-suites has an electric shower. The isolator for that... in the built in wardrobe adjacent to, but outside the bathroom - a 32A cooker type switch (fine in terms of load for the shower in use, but less so in terms of easy access).

Once I'd fixed all the earth issues etc, I replaced the old wired fuse board with a modern consumer unit and RCBO's.

I genuinely think people fail to understand how dangerous mains electricity is because you cannot see or hear it unlike gas and water.

The little old lady who used to live next to me would obsessively switch off mains sockets to stop the electrify leaking out.

My wife still does. I value the peace and serenity when I don't bring this up.

Crikey.

Indeed.

I'm not an electrician, but wiring things into a live installation definitely sounds like something I would not like to do.

Electricity is dangerous enough when you have time to manage it properly. Plugging things in to live voltage is just asking for trouble.

50v is considered "non-hazardous" if touched. However, you do still have to worry about short-circuits (as is shown here) and make you take off rings and the like if you want to keep your fingers.

I remember someone at Uni who was wearing a metal watch strap which came unclipped as he was working on a 200V DC motor. It fell across the terminals, he was very lucky to get away with only a bad circular burn right around his wrist.

DC is nasty. He was *very* lucky.

Yeah. That sounds very safe to me.

"The other trick was that the installation had to be done while the power was live. It was, after all, a telephone exchange, so shutting it down for any length of time would represent a great pain in the proverbial."

Now that's not what I call a safe environment.

"It also blew the main 100 amp fuse – cutting power to the entire exchange."

How many amps were running into that station ? Blowing a 100amp fuse is not a trivial affair.

I can't understand either if the 50V was the correct value but at any rate the unintentional electrical spark fest must have been nerve racking to say the least.. It probably made a nice bang too..

And I still can't understand the need for the second 16 amp fuse ?

A 100A fuse will typically take 150A for many, many hours before failing (all depends on the fuse's characteristics). Higher currents give faster fusing. For example, a 100A BS3036 fuse:

430 A - 5 s

1200 A - 0.4 s

2800 A - 0.1 s

100A at 50V is only 5KW and it will have been pulling less than that under normal circumstances.

Yes, 50V is standard in these environments. Technically -48V to -53V (the positive terminal is grounded as this polarity reduces corrosion) That's made up of 48V nominal on batteries (easy as 4x generic 12V batteries, but usually ended up as 24x big 2V cells) and the charger running a bit more than that to maintain the charge. This is also the standard that influenced PoE voltage.

The 2 x 16A fuses will have been 2 drops into the rack. Probably for a bit of resilience in case some of the kit shorts out. Possibly because the rack needed more than 750W and 16A fuses were standard on drops.

--- took out the comms link up to Darwin --

So it was powering a repeater as well as the local lines.

He managed to (ahem) avoid the short end of the stick

I saw a big spanner mounted on a nice wooden plaque in an exchange - the spanner was melted through the middle into two pieces.

The story was that a tech walked along on top of a cable way and banged his head on a concrete beam, causing him to drop the spanner onto the main power bus bars. The whole exchange went down and back up.

Such an event made much money, since everyone had to call again and got dinged for a new call.

Pretty sure I know THAT Mick.. Yes 50V is the norm and a dead short = a bright flash and a good bang

In the first place I worked, we had a goods lift that could lift three cars at a ttime onto the roof, so it used a lot of juice.

One day we lost power to the lift, so the maintenance guy came out, checked things over and decided we needed a new fuse, he pulled one from his bag, it was the size of a beer can, rated to 400 amps.

Wearing very thick gloves and weilding a sturdy pair or tongues, he removed the old fuse. The belt of lightning as he put the new fuse in place was a sight to behold. It also took out all our power and the neighbouring units either side.

Maybe he should have checked the power was off first...

Maybe he should have checked why the first fuse blew, before just replacing it.