Replies

1. 
   4Lorn1 | Apr 21, 2002 01:45pm | #1

   If it is not damaged or overloaded knob and tub wiring should be relatively safe from a fire perspective. Knob and tube (K&T) wiring was designed to work with the insulation technology of the day.

   Wired were insulated mostly with natural rubber. It was known that this material would break down in a few years so K&T was designed to remain safe without depending on this insulation. Porcelain insulators, knobs, hold the conductors away from surfaces and the tubes sleeve the wire where it must go through a drilled hole in joists or other members.

   I generally prefer to remove K&T wiring and replace it with modern materials If the K&T is not damaged, tampered with or overloaded it is fairly safe in terms of shorting out. The problems I have with K&T are that over time poorly done additions, repairs and  general abuse become more common. Also K&T does not commonly include a ground wire.

   There is an old saying about poker. If you look around the table and you can't spot the pigeon.... it's you. The same with grounding. If you look for a ground and can't find one your it. The ground conductor carries stray currents so you don't have to. Very important that.

   As for your question. Arc fault breakers protect against shorts too weak to throw a normal breaker but likely to build up enough heat to cause a fire. This is not normally the weak spot for K&T but it certainly won't hurt. If you are unable to do a rewire I would recommend adding GFIs to increase personnel safety.

   GFIs work by comparing the outgoing current vs the return current and tripping in a few milliseconds ( typ .007 seconds response time). In a proper circuit the outgoing and the return will be the same. If current were to be diverted by a wiring fault through you and say to a sink the return current would be smaller than the outgoing and the GFI should trip.

   Having a GFI upstream of a receptacle is also one of the few ways that allows you to use normal receptacles. It won't make the unconnected ground work, surge blocks won't function, but it will allow you to avoid having to hunt down two-wire receptacles and use adapters.

   Until you can save up enough for a complete rewire, they are seldom cheap, add GFIs and if you wish AFBs. The AFB can only help. Also talk to your local electrician and ask about adding, upgrading or rewiring the higher usage circuits only. Circuits that feed kitchens, bathrooms ,laundry and any circuits used for space heaters or A/C all tend to carry higher amperages for longer times than any other circuits. Do these first.

   While your electrician is sizing up the job have him inspect the K&T for damage, etc. Do not allow any insulation of any type to be added on top of or in contact with the K&T runs. When insulation is added moving in an attic becomes a matter of feeling for joists to step on. With delicate, potentially uninsulated and live wires hiding in the insulation and assorted tradesmen leaning on potentially grounded pipes and ducts with sweaty hands while groping for hidden joists you may be faced with piles of unsightly corpses cluttering up your valuable attic space. 

   Even though these bodies rapidly desiccate and become mummies they do smell for quite some time, and you thought they smelled bad when alive, the problems aren't over for they may have, very rudely I might add, may have managed to damage the wire runs enough to cause a fire. Firemen tend to ask embarrassing questions about mummies found in attics. Most reputable insulation contractors will not insulate over K&T.

1. 
   rjw | Apr 21, 2002 03:05pm | #2

   4LORN1 said:

   "Wired were insulated mostly with natural rubber. It was known that this material would break down in a few years so K&T was designed to remain safe without depending on this insulation."

   I'd never run across that info.  I was under the impression that it was a heat dissipation issue, but can't claim any particular authority on that.

   4LORN1, what was your source for that.  I see plenty of the stuff and usually the insulation problems are where there has been excessive heat: e.g., overlamped fixtures.

   I'm always ready to lear, though.

1. 
   4Lorn1 | Apr 27, 2002 06:38am | #6

   First. I'm talking about the mother of all wiring systems. Knob and tube wiring was an adaptation of the method used to rig wire on between power poles. It dates back to the 1910s with its height around the 30s. The actual system used, with some updates and the year these updates became available and widely used, varies from region to region.

   The problem is that chemical technology hadn't developed many plastics to use as insulation. Bakalite and phenolics were available but both of these are thermosetting and stiff. This makes them difficult to apply to the copper conductors and if you do you end up with a cable that can't be bent. This construction was used in some industrial buss bar systems but it was impractical for house wiring.

   The alternative was rubber. First they try natural rubber, same stuff as latex gloves and condoms, but it doesn't work well. It's soft, easily torn, susceptible to ozone, sunlight and petroleum products ( Don't use Vaseline with latex condoms. The latex will turn yellow and brittle in a matter of minutes.)

   The first commercial insulation was the more rugged vulcanized rubber. Same stuff as tires. After a short time a problem developed. The same sulfur that transforms latex to vulcanized attacks the copper of the wires causing a layer of green copper oxide to form around the wire. The solution was to tin the wires before coating them with vulcanized rubber. Some people have been needlessly scared by seeing the silvery tinning, a coating of tin and lead, and thinking they have the more hazardous aluminum wiring.

   All of these assemblies, I'm still talking about a single wire, were commonly covered with cotton windings. Typically these were natural, unbleached, cotton and tan in stripes on the neutral and natural and brown or black on the hot. If the cotton survives the years the tan gets darker. This can make telling the neutral from the hot problematic. This is also true of early romex.

   The improved vulcanized rubber on tinned copper with cotton windings conductors were state of the art for many years. The problem is that vulcanized rubber, like automotive tires, are still damaged by sunlight , ozone and heat. Over time it becomes stiff, brittle and eventually turns to powder. This degradation is less in walls , crawl spaces and other places with limited heat build up and air movement. I have seen knob and tube (K&T) wiring with the rubber still live, flexible and sound after 60 years or more. On the same house I found wires, in the attic, that had no insulation remaining even though the remains were still visible. Even without insulation the wires were kept safe by the space provided by the porcelain.This is why knob and tube wiring, to the extent practical, is made so that it is suspended by porcelain insulators or sleeved in porcelain tubes.

   While some early systems had small porcelain insulators that were run on the surface of the wall as were the devices, receptacles and switches. A more modern version of these devices are still installed in agricultural buildings. Soon enough styles changed and wiring became less of a status symbol. People no longer wanted to advertise that they had electricity and so wiring was moved into the walls. The weakness in K&T then appears when it must be threaded down walls and into boxes. The solution to this was loom. A flexible woven manila tube treated with tar as a binder. This was used to sleeve the individual wires down the wall and into the metal switch boxes. 

   You are correct that the limiting factor in wires is the ability of the insulation to withstand the temperature rise caused by the wire carrying increased amperage. K&T, at least when loom is not necessary, is less sensitive to heat build up. When these early wires got to the junction box and especially in fixtures it tended to break down. This spawned an entire industry of rewiring fixtures the down leads in houses and eventually led to equipment grounding.

   The sources of this information include :an early, 1940s, copies of the electrician handbook. A 1930s copy of an early NEC.  The thing has about 50 pages. An early wiring manual produced by the IBEW. Discussions I have had with electricians that ran wire in the 30s. My personal experience with repairing and rewiring houses built in the 20s and 30s. Some extra research done at the library to try to find out how and why houses were wired with K&T.

   Some things I learned have made rewiring a little easier. This might not answer your question but mentally reviewing this information was fun for me. You asked the time and I build you a clock.

2. 
   desirednam4 | Apr 23, 2002 12:47am | #5

   4lorn1... that's the best d a m n post I will ever f r i g g i n read in regards to electric yak!!!

   You are the electric man!

   Lakeside...On the mountain, near the stream,aj Builder of Fine Homes & Tennis CourtsAnd featuring; Great Camps of the Adirondacks

2. 
   CPopejoy | Apr 21, 2002 10:57pm | #3

   Prussing,

   AFCIs will probably help with one potential K&T problem--bad splices made in modifiying the wiring.  If an improper splice is arcing (spitting sparks), the AFCI should open the circuit.  Original splices are almost always well-made and are nothing to worry about.  Splices made in adding on to K&T are usually not enclosed in electrical boxes like they ought to be, and are often poorly made.  AS far as the wire insulation on K&T, the earlier commentor is right, a good original K&T system is designed to be safe even if the wire insulation is compromised.  The exception to this is in fixture boxes, whre the wires are in close contact.  Heat from the light accelerates insulation degredation, and shorts can occur especially if the wiring is disturbed.  I have seen a lot of 80+ year old K&T wire and generally the insulation is a really good shape (if it hasn't been buried in building insulation, chewed by rats, or damaged by people mucking around on the attic). 

   AFCIs will also protect (well, are designed to protect) against arcing in an appliance or light and it's cord.  In fact, I believe that this is the primary hazard that AFCIs are intended to reduce.  They also protect in-wall wiring, of course, against arcing.  Or rather against the patterns of arcing that the little AFCI brain chip has been designed to recognize).

   GFIs will offer protection against shock to people, which is an issue if you're using a three-prong appliance with a cheater (adaptor) on an original, non-grounding K&T system.  BTW, GFIs are designed to open if a current of 6mA flows for 6 seconds or more.  This will protect most of the population from a shock that would pose a serious electrocution danger.  I have the special gear to test GFIs (new and existing) to verify this time-current trip point.  Old and heavily used GFIs sometimes test OK with the GFI test button, but do not meet the time/current critera.

   My suggestion to you is to rewire the whole place as you can afford it, replacing the circuits first that see heavier current use.  The issue with K&T is heat build up when the wire is buried in insulation.  Heat build up happens when there's a heavy load in the circuit.  I've seen framing charred from the heat of a K&T wire buried in insulation on a circuit where an electric space heater was regularly used.  The only thing that prevented a fire was that the stud bay was so well sealed, not enough oxygen was available to sustain combustion.

   There's no cheap and easy way to make K&T installation really safe, if there's insulation in the attic or walls.  And who doesn't have their attic insulated these days?

   Cliff

3. 
   user-44383 | Apr 22, 2002 05:55pm | #4

   Thanks everyone for the information.  This will help me sort out the priorities in this home.  I'd better go into the attic and see what is up there in terms of insulation around wiring and possible organic matter.  I have noticed an odd smell every once in a while......

   Seriously though, thank you for the well thought out and thorough response.