Home & Garden Columns

Wiring to Avoid: Porcelain Fuses and Knife Switches

By Matt Cantor
Thursday July 16, 2009 - 10:39:00 AM

I’m a sucker for old things. Seventy-year-old cars, 100-year-old tools and Fiesta-ware pitchers. Actually, much of the reason I inspect houses for a living is my love of old things. Yes, there are those bothersome days when I have to inspect a brand new 4,000-square-foot house, but I generally manage to conceal my ennui. (And then there’s the money…)  

Among the old features that fire those endorphin-generating neuronal pathways are the tiny doors that gave access to shower drains, milk doors where your children’s real father would leave the butter and cream, and funny little wooden fuse boxes lined with lots of real asbestos (be sure to smoke lots of cigarettes before you play with it). 

My love of these particular aspects of our early 20th century houses is truly warped. I confess to having a small collection of “knife” switches (you know… “It’s Alive!”) and lots of porcelain fuse and bulb holders. The effort applied to manufacturing these simple electrical components was astonishing compared to the modern electrical panels that handle 125 amps and have covers where the screw holes don’t line up. 

That said, my ardor does not keep me from appreciating the mortal threat that some of these elements present to the unfamiliar users. Especially the naive 9-year-old. And given that I still see a fair number of these very primitive elements in the houses we’re all living in, it’s worth it to point out a couple of major threats that you might still be living with. 

Among the most common, is a condition called Neutral Fusing. Prior to 1928, most houses were wired with glass Edison fuses on both the hots and neutrals in your electrical system. Now, don’t fret, I will explain about hots and neutrals and it’s quite simple. When you look at the little surprised face on an electrical outlet, you see two slotted openings and one surprised roundish mouth. The two slots are hot (+, positive) and neutral (-, negative). The mouth is an extra neutral, what we call a ground. The distinction between neutral and ground is pretty much a protocol since they both end up connecting to the ground, literally. 

So, when we wire a house, we want to protect the wires from getting too hot (literally) and so we put temperature-sensitive devices (fuses) in the path that power must pass through (the hot wires) to get to the outlets, lights and everything else. What they used to do was to put another one of the same devices (a fuse) on the path back from the devices (the neutral path) to the earth. This meant that some of the time the fuse that blew would stop the power from getting back to the earth but would leave electricity still present on most of the wire on that circuit.  

So, the neutral fuse has blown. This keeps the circuit from being completed and the devices (electric foot massager, lava lamp, kiwi juicer) from working. So what do you do? You meddle with what you think is a dead outlet, lamp base or other part of the wiring system. If you’re like many resourceful, helper types over the last century or so, you know just enough to start taking things apart (that you assume are dead) and you touch a live metal part and get shocked. In fact, since you’re the only possible path back to the ground at this point, you might just get a very serious or deadly shock. 

The folks who write electrical codes figured this out and, as I mentioned, in 1928 they eliminated the practice of neutral fusing. They said go ahead and fuse where the power enters the wiring system but leave all neutrals and grounds (which are essentially the same thing) connected to the ground all the time. 

If you look at enough of these things, you’ll see the modifications that were made in the years that followed in which they took the same fuse holders and soldered pieces of copper across the path to eliminate the fuse and then, just a year or two later, they all change and we see neutral buses (which has neither to do with gender nor public transport), little mounts where all the neutrals gang together and head off to touch the earth. 

If you’re in a house from before 1928, take a look in that very old fuse panel and you may be able to discern the layout of neutral fusing. These usually include porcelain fuse holders screwed onto a wooden backing in what is often a wooden cabinet in a hall or closet (I’ve found them in kitchen cabinets, too). Two wires will connect to two screw terminals on one side of a four, six or eight fuse base and then an array of wires will come off of either side. Ultimately, a tester is needed to determine if these two wires produce only 120 volts. 

The two wires will produce 120 volts because only one is hot and the other simply connected to the ground as a return path. If both are hot and part of a 240-volt system in which the two are “out of phase” with one another, we’ll get 240. Two hundred and forty volts enables us to heat stove or dry clothes. 

You can find out of you have 240 coming into your house by looking at the service drop outside your house. If three wires come down from the main power pole and connect to your house, you have 240 volts. Be cautious, there are often three being provided by the utility company with one of the three being folded back and left for the future 240 service that you don’t yet have (you must spy with your little eye very carefully to see the one that’s doing nothing).  

We still have houses around that have only 120-volt service and many of these still have nothing more than a single pair of 30-amp fuses in a compartment in the side of the house. Or even hanging on a wooden board in the crawlspace near the front of the house. These often feature an open knife switch of the kind I mentioned earlier, a meter and open wiring leading to more fuses on the board or in the house somewhere. These installations abound with places to rest your hand on energized metal parts.  

What were they thinking in 1915? Well, they weren’t, really. Hindsight is always 20/20 right? It was incredibly exciting having the new “electric” light in the house and the danger hadn’t manifested yet in the form of any real number of deaths. Safety features always take time to develop and, for better or worse, many of us still live in houses just as they wired them in 1915 or 1925. Imagine what they will say of us in another 100 years. What sorts of wild, mad risks are we taking that we saunter past on a daily basis (freeway driving, unbridled energy use, voting Republican)?  

So, do as I do, keep those old porcelain fuses bases and Frankenstein knife switches for everyone to ogle and laugh over….disconnected on your mantelpiece.