me again with electrical question

[Texas_Ranger]

Lightning rods are mostly to keep your house from burning down if lighting hits the roof but they also protect antennas and the likes from direct lightning strikes.

Surge arrestors are electronics (basically variable resistors) that connect to your power lines and filter out over-voltages caused by lightning hitting the lines somewhere else. Basically at line voltage they just sit there doing nothing but if a lightning strike occurs and the voltage goes above like 500 Volts instead of the normal 120 they start conducting electricity to ground, protecting your equipment and tripping the main breaker. You can also get surge arrestors for phone lines, handy to keep your DSL modem and cordless phone from blowing up.

[phil]

in fixing old radios and things, we usually use an isolated power supply. It's basicly a 1 :1 transformer so the voltage in is the same as the voltage out. There is no electrical connection between these two coils. , the energy is transferred by magnetic flux. since the power that can be transferred is then not hard wired to the mains it is isolated. the isolation transformer used on a bench to power a radio or test gear is usually around 2 amps or so. If you try to draw more than that the isolation can't deliver the extra current.
You'll see them used in video arcade machines and other places. one could be used on old knob and tube circuits and that would deliver the same ( 110- 120) voltage to the outlet but current could then be limited to whatever the isolation transformer is sized to. On the radio bench it is safer for the technician, also the bench is not normally grounded as grounds like a steel bench can present their own dangers. Using an isolation transformer is even more important when working on hot chassis radipos and they were common in about the 50's . they relied on plastic knobs to isolate the user but if you pull the knobs off , or work on the radio chassis without knowing you can get a surprising shock. these radios usually didn't' have polarized cords and restorers will often update the cords to try to prevent the chassis from becoming hot at least when switched off. The narrow of the two prongs is the hot and the wide prong is neutral and if you switch the hot side then there is no power past the switch when it is turned off.

you can make an isolation transformer if you can get a hold of two salvaged transformers. say if you came across two that would step up or down the current, for example one for changing 110 to 220 would be a 1:2 transformer. you just hook them back to back and then it would take the power from 110 to 220 and then from 220 back to 110. you might even be able to use a couple microwave transformers if they are isolated. even though they put out a high voltage, the principle works the same if you hook them back to back you;ld just need to be cautious that the high voltage leads were insulated properly for the higher voltage.

one needs to note that all transformers are not isolated. they can transform power by tapping the coil and that's called an auto-transformer and these are common so you can't assume any transformer is isolated.. to check you can just see if there is any electrical connection between the input and output leads. sometimes the coil is tapped so you can get various voltages but those don't isolate the power so they aren't suitable for this.

I think you could improve the safety of a Knob and tube lighting circuit by installing an isolation transformer near the panel and between the service and the outlets. the amperage that can possibly be delivered would then be limited to the size of the transformer used. since there is no ground reference it becomes safer because if you did touch one of the hot wires and a ground , then you still wouldn't be part of the path. maybe that would be a solution for someone that doesn't want to change wiring but wants the circuits to still be safer than they are now. Maybe if you have a museum and you really really want those old two prong outlets. It could be an interim solution for some. when you upgrade just pass the transformers on to someone else. I'd still consult an electrician to make sure what you are doing is safe but I think most would agree it is safer than doing nothing and at least this is a change that can be made without replacing existing circuits.

Im not sure of the amperage of this one but just as an example :

http://www.itm.com/product/bk-precision ... oCKaHw_wcB

Phil

[Texas_Ranger]

Transformers aren't automatically current-limiting, you'd need a fuse in most instances. Supplying extended wiring systems by isolation transformers is a pretty complex thing too because as soon as you have one fault on the secondary side the system is no longer isolated but the fault won't be detected by an upstream breaker or GFI.

The point of any electrical system is this: a voltage is a potential difference between two points (or wires). 120 V between hot and neutral doesn't say anything about how many volts to ground (e.g. a water pipe or a piece of steel hammered into the ground) you have. Could be 130 V from one wire to ground and 250 from the other or 20 from one and 140 from the other or even 5000 V from one and 5120 from the other.

However, power companies like clear definitions so they tie one wire to ground so it is always at 0 V and the other at 120. That means one wire, the neutral, is (theoretically) safe to touch while the other is dangerous. If you touch it while another part of your body is grounded, current will flow through your body and possibly kill you.

An isolation transformer removes that link to ground, both wires are "floating" and can be considered hot. However, if you touch one of them it just becomes the new ground reference via your body and nothing happens to you. That's the "first fault". It doesn't have to be your body, it can also be a live wire touching a water pipe or something. Now as soon as you've got a ground reference on one wire, touching the other immediately becomes dangerous, this is the "second fault". There are complex system that ring an alarm when the first fault occurs, typically used in ERs and other sensitivc places where you absolutely don't want the power to be cut in case of a fault. This isn't something you'd install on your average old lighting circuit and besides those systems need a "ground" wire although that's only connected to the monitoring system rather than to an actual ground rod.

In all honesty I'd be much more worried about arc faults and fires than about people getting shocked by K&T wiring! IF there are no shared neutrals you can have AFCI breakers installed. An electrician should be able to figure that out by disconnecting each circuit's neutral at the panel and measuring whether the neutrals at the lights and sockets are still connected to anything.

[phil]

interesting reply Texas. I think you have some good points. I suppose the secondary could be arranged to trip off the main power via an overload breaker , would the current not be limited to the amount of power the isolated transfomer can deliver? It does get more complex when you speak of this alarm system to detect a fault in the secondary.

[Texas_Ranger]

The transformer would need overload protection, e.g. a breaker, fuse or thermal cutout, yes. Otherwise the transformer could simply burn down. There's nothing to prevent you from protecting your house wiring using smaller fuses or breakers though (except I think anything smaller than 15 A is very uncommon in the US) so you'd save the cost for the transformer.

[phil]

I guess the old screw in type glass fuses are still an option, and you could probably get smaller values. at least they won't stick and burn up wiring as a result. I suppose you could just put a fuse holder in the box the transformer is mounted in so it couldn't burn up. I'm not sure how that would play into wiring code.

[Texas_Ranger]

AFAIK larger setups with isolation transformers are tricky to put it mildly so I'd simply drop that idea. If you were seriously worried you could look into smaller fuses for the entire circuit, although anything smaller than 15 A really limits usability at 120 V.

[phil]

I originally figured that since it's common practice to use isolation transformers for safety at an electronics bench that one could just use one to keep an old knob and tube circuit live but Texas you brought up some points about how such a system could trip out. The transformer could be fused, that would limits it's current carrying capacity but you may have brought up some points that it may be impractical as a general purpose fix for suspicious old wiring.

I replaced all mine so I'm not concerned but maybe there are some practical solutions for people who want to keep some Knob and tube alive, as an example maybe just for lighting. My electrical inspector didn't even care if I pulled them out but asked me to just clip them short to make sure no one ever reconnected them. I think He could see from the complete re-wire that I didn't' need them so wasn't concerned.

I like playing with old electrical apparatus because it fascinates me and I love to see how old meters, test equipment, radios and related parts as they are often just so well made and beautiful. Ive got lots of really neat examples of meters for checking batteries and capacitors and tubes and things. decade bridges, early tubes, lots of it is only really for static display.
I use some for working on old radios but a lot of it I just keep for display purposes. In some instances like in checking older radio tubes you need the old test equipment so I have some that I actually do use. Ive gathered what I could by studying a lot of info repair books but they get dry and confusing pretty quickly. When I get stuck on a restoration then I sometimes have to learn more to dig my way through but that's part of the challenge for me. I just bought a radio from the mid teens. I posted a pic of a very similar one under the antiques section. I'm mostly focusing on repairs to my old house but once I get things a bit more sorted I plan to pair down some and restore some and continue with the hobby. Its just fun and a lot of the collectors are really nice people. Its good to learn what I can particularly from the elderly members that still really understand this stuff. Many have a limited time to pass it on before they can't explain it. I did sell some to a younger member and he seemed to have the bug. We've been having some public sales and that is generating a few newer members. As it is most have stuffed houses and there really isn't a lot of interest from the younger generation to pass the torch to.

[Texas_Ranger]

IMO the best thing to do with ancient wiring of any kind is to thoroughly check its condition especially where exposed (i.e. in boxes), possibly even have it meggered (basically a resistance check from conductor to conductor but at a very high measuring voltage rather than the 1.5 V of your average digital multimeter in order to determine whether the insulation is still electrically sound or close to shorting), avoid any light fixtures that could heat up the wiring (i.e. only use pendants) and possibly use the smallest fuses you can get by with, reducing loads to only a few lights.

[phil]

that's good advice. Ive worked with electricians using meggers before like when we are looking at an old motor and the wiring is old and suspicious. I have often thought if one were to unhook the wires at the panel and then go unhook any loads and intentionally short the circuit at various points one might be able to take resistance readings near the panel to determine if there is any resistance like bad connections which are hidden from view for example.

an old trick for fixing unobtainable antique radio transformers which have a broken winding is to hook them up to a high voltage like 1000 volts but only for a split second. the theory is that sometimes you can get them to re-weld the ends of the broken wire inside the windings. Its hit and miss obviously but some have claimed to have success. I'm not suggesting doing this on house wiring but Its a good point that you could have a ugly blackened connection that might melt down under load but when you check it with your little multimeter you might not see the resistance since it's welded itself even though the wires are all burned and unsafe at the actual connection , so I'd assume the resistance check would not be reliable but if you did see high resistance you might have reason to look deeper. of course there should be no condition that should produce high resistance measuring from the panel to the short and back to the panel.. of course excluding expected losses from the resistance of the wire. If you saw normal resistance it wouldn't necessarily rule out that you could still have a weak connection but if you did find high resistance then you know something is causing it.