How much current safe for 30m extension?

the first logical thought is repair them rather than replace. Lead acids are often repairable.

Quick repair, doesnt always work: rinse muck out of cells fill with new acid charge, monitor acid conc and adjust as required

This wont fix all cells by any means, but many it will. Those it wont in some cases may only need replacement plates and acid, which can be made much cheaper than buying new batts.

Also they can often be run happily at 24v, if not in all cases. If yours could, it may be that your present cells would do that as is.

What capacity at 48v is it on?

i've no ida how that fits into this

well how much current are you shifting?? And why cant you power your pbx from whatever power source it uses now? You havent given us nearly enough information.

standard uk T&E should do that, but shuold be insulation tested to the required v first. Costs around =A312 a 100m reel, depending on copper size.

rf transmission co-ax is rated pretty high... and its concentric. T&E's probably far more practical.

10A

NT

Hmmm... if you do it, just connect the PBX directly to the

480 volts. Just think how much faster the phone calls will be at 480 vs 48! :-)

I think installing a half mile of # 4 might be "politically" cost prohibitive, regardless of the technical merits. And I doubt it would be a good financial solution. How many times can you replace the batteries for the cost of digging a

1/4 mile trench, and installing conduit, cable, fittings, transformers, etc ?

Ed

For that money you coud buy the PBX a UPS all of its very own!

One question to somebody presumably residing in the western colonies.

What's a "weed wacker" ?

It's (I think) a trade name that has become a generic term for a gas or electric (with or without cord) powered lawn trimmer/edger:

They cut by flailing around the end bit of a spool of thick filament of extruded nylon, and are thus unlikely to cause serious injury. When the nylon wears down or breaks off you typically dispense a bit more by thumping the business end (as it spins) onto a spare bit of grass.

We have one with a cute little gasoline engine. I assume it's a trade name because when I inquired about one in a B&D outlet store, the clerk pretended not to know what I was talking about.

Is that what you'd call a "Strimmer"?

Best regards, Spehro Pefhany

"Peter A Forbes" wrote | >Those poor souls that put a hundred feet or so of 18GA (about 1mm sq) | >extension cord on their weed wackers

It's a different language isn't it :-)

| >soon find that not only does it run slow, but the motor overheats. | >So we have extension cords that are 16 or 14 gauge, and can | > handle the extra current. But people are too cheap to pay double | > for the heavy duty extension cord, so they end up eating | >their money up in burned out motors. | Why use anything as small as 1mm sq cable???

If it's in Wall-mart, they'll buy it.

Owain

Hey, a hedge trimmer will cut the cable cleanly in half, no matter what size it is! Ask one of the ladies at work, she has first-hand experience!

As for heavier cable, I know that, but the average consumer doesn't. She uses an 18 gauge extension cord with the cast plastic socket from the wall to her ironing board, and wonders why she smells melted plastic when she irons! "Gee, I swear that when I plugged it in, the socket wasn't all brown and charred." Doh!

The guy has what's at hand and just plugs three 25 footers together to wack his weeds. The plugs and sockets also drop some voltage because of their resistance, so he ends up with a burned out motor after it ran a couple hours at 85 to 90VAC. Doh.

No. These cells are 4V each, 12 in all, each is sealed. Well, except for the one cell that's split open because of internal pressure. :-( They must be replaced, not repaired. This is a phone system, where it has to be online during emergencies. Batteries that are working fine, but over 5 years old are considered unacceptable and must be replaced.

This is *not* how you run a battery backup system! The idea is to start out with fully charged batteries, so that when the power fails for a long period, the voltage of the batteries may drop to 40V or less as the cells discharge, keeping the PBX online. If you start at anything less, even 36V, you're not going to have any discharge time before the PBX system crashes. It would be essentially worthless.

The rectifier can put out a hundred amps at 54VDC, to charge the batteries while the PBX is also running. That must be 6 or so kW input. So I would guess that it's 208V at 30A breakers. But with no batteries, the load would be more like 3.6kW.

The UPS in the computer room is 150kW, easily capable of handling another 6 or so kW.

The batteries and rectifier are a "UPS" to protect the PBX against failure of the commercial power. That's what it's being fed from.

Assuming that it could be pulled 1300 feet. Probably not without damage. Heavily insulated cable would be needed. Probably double jacketed.

RF transmission coax is typically copper clad steel wire center conductor, which would have much greater I^2R loss than pure copper cable at 50 or 60Hz. That would be unacceptable.

Thanks, but I've never heard of T&E here in the U.S. I don't know what that acronym stands for.

[snip]

NT

In message , "Watson A.Name - \"Watt Sun, the Dark Remover\"" writes

Twin and earth

... colonials !

Last time we had a major failure of the batteries and rectifier it cost over $15,000 to get it repaired. Obviously that was before 1996, when the present batteries were installed. So the price would be somewhat higher today.

The spare conduit is already in place underground, all that is needed are the power cable and associated switchgear.

We regularly have the 4160VAC cabling HiPot tested on campus, and on occasion a section has to be replaced. Y'ever see this stuff? It's

600MCM, with heavy insulation and shielding. The copper is as big as your thumb, and it's expensive!! I have a footlong section of it on my desk, and when someone asks what it is, I tell them it's high speed datacomm cable! ;-) WOW! Really??

I googled for that and this URL says that 600MCM is 29mm. Fat pipe! Imagine all the data you could shove thru it!

[snip]

"Strimmer" to us.

That's probably what will happen. But the computer room's UPS is backed up by a 300kW Caterpillar diesel generator, with 300 gallons of fuel in the tank. It's supposed to run for at least 8 hours, so it does have an advantage. We've had power outages that last for hours, so having that much battery backup on the PBX would cost a lot more. But we'll probably buy a couple APS 2.2kW UPses and put the PBX system on those. They should last for close to an hour.

wrote

Where are you from? Ever heard of Google?

This URL shows a pic of the business end of a weed wacker. Traditionally they use monofilament. There are both electric and gas engine types, but the noisy gasoline engines (and leaf blowers) are sometimes banned from some communities.

\"Watt

Yeah, we call it ground here stateside. And it's Romex.

And it's AC line in every schematic I've seen from the American mfgrs the likes of HP, Tek, etc., not "mains". Mains are those wires up on top of the power pole.

Thanks T.A.!!

(That's Thomas Alva, for you right ponders.)

I don't know of any 4 volt rechargeable cell chemistry.

I expect that your "4 volt cells" are actually two cell lead-acid batteries, with each cell good for 2.1 volts.

If you're going to use transformers at each end of the link, you can make one of them tapped, so you can select an appropriate tap to compensate for the voltage loss in the cable.

I would say 2.5kw at 30M and around 2Kw at 60M. Its not gonna make much difference(resistive load) if the cable is protected by the correct fuse in the plug.

The big problem is going to be earth loop impedance, I know it sounds trivial because it will work fine, but you did quote a PSC figure of 120A......must be a TT instalation. Please make sure you have a RCD on this circuit..

Regards, Paul Miller iEng MIIE (elec)

replaced.

replaced.

I expect you're right. Two cells in a case.