Old K&T component/fixture identification

Came across an old (no longer connected) component of what I assume was the original knob and tube electrical wiring. Client asked me what it was and I didn’t have a definitive answer (had a few guesses). Anyone here know/point me in the right direction?

Old phone line equipment.


I think it is a lighting resistor

I am curious if that back pad is asbestos? Just throwing that in :wink:


Here they call it an electric and telephone lightening arrestor


You the man. Thanks!!

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That makes sense.

There are modern versions of the same thing, still in use.
In fact if there’s a land line going into the house, it’s a defect if that’s not present.
Look for a ground wire to the telephony looking grey box.

These days the suppressor is built into the “network interface device” or NID. If that and the cable TV do not have a shiny copper ground wire, it’s a defect.

The OP’s pictured back pad is almost certainly asbestos. In my area that’s $700 just to dispose of right there, and requires EPA registration.

You’re correct but the connection is not required to be bare copper, it can be insulated. Around here the telco and cable companies use insulated conductors.

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Now that’s shocking severe overkill for a few grams of asbestos. I still find asbestos frequently in exhaust pipe insulation, boiler pipe wrap, internals of old boilers, and complete house siding.

It’s a clear case of regulatory overreach making “doing it right” so hard that it increases the incentive “do it wrong”. Our local household hazardous waste systems for example won’t take it (they take batteries, mercury bulbs, paints, chemicals).


A state ID number is required since asbestos is not regulated by the United States Environmental Protection Agency (U.S EPA) as hazardous waste under the Resource Conservation and Recovery Act (RCRA). Asbestos is considered a “non-RCRA” hazardous waste if it is friable and contains one percent (1.0%) or more asbestos as hazardous waste.

Sometimes a bit of asbestos can find its way into a double bag, and accidentally be left on a truck, where it might end up being combined with some other asbestos… in which case a few extra ounces is essentially free.


Interesting facts

Doubling a small number seems impressive:

Apparently it’s still used here:

For making chlorine (nasty stuff to make, use, transport, or spill).

Throughout the past few years, the chlor-alkali industry has used most or all of the asbestos imported into the U.S. In 2016, the EPA noted that 340 metric tons of chrysotile asbestos were imported and all of it was consumed by chlor-alkali plants.

Wanna bet that if chlorine production moves out of country, a few years later there’s a bad spill while importing it?


It is a lightning arrestor. They mainly gave people a false sense of security. They weren’t completely ineffective but they didn’t offer much protection either.

A common characteristic that differentiates a lightning arrestor from a surge suppressor is a spark gap. Most lightning arrestors have a spark-gap, while surge suppressors do not.

The lightning arrestor depicted in your photograph has two resistors and a spark-gap in the small housing at the top. The spark enclosure is typically vented to release hot gasses.

The rule of thumb is that under normal atmospheric conditions, it takes about 30,000 Volts to jump over a spark gap. Once a spark (arc) has been established, the air becomes ionized, thus allowing continued current flow and will clamp the Voltage. The resistors limit the current. In a lightning strike, the current will typically flow for only a fraction of a second.

By comparison, surge suppressors typically employ a variable resistance, such as with an MOV, to clamp the Voltage. There is no spark gap. Surge suppressors, therefore, can be designed to operate at any Voltage and are immune to changes in atmospheric conditions.

Lightning arrestors have been used since before the days of buildings having electrical wiring. Another characteristic of lightning arrestors is that they usually redirect the signal directly to Earth instead of creating a short-circuit. The style depicted in your photograph uses a spark-gap to create a short circuit.

As with many terms in the electrical and electronics fields, people have appropriated terms for marketing purposes. Some small surge suppressors are sold as lightning arrestors, but that’s a misnomer. Strictly speaking, they are surge suppressors. The device depicted in your photo is an actual lightning arrestor (because of the spark gap), albeit a relatively ineffective one.


Combining both is best: I have examples of doing so that actually took a full lightning strike and protected the equipment. The spark gap takes the brunt, the MOV and Zeners take whatever gets past the spark gap.

George - many thanks for the detailed reply! Appreciate you taking the time and sharing your knowledge.

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This device was not used for knob and tube it was for telephone systems.