When a plumber has to cut a water main service line either by cutting a pipe or disconnecting a fitting an unseen danger lurks. You don’t often hear of it but plumbers face an electrocution risk when they cut the main service line. A series of electrical faults in many cases can cause the water service to be carrying deadly electrical current without any knowledge until the pipe is cut and a shock results.
Testing to see if the pipe is carrying current is not understood by many. If you try to test for this with a meter testing voltage you probably will not get a reading as voltmeters depend on a voltage differential to get a reading. I recommend using two things that could save your life. A Non-Contact Wide Range AC Voltage Detector Probe will alert you to this problem, and if AC Voltage is detected an electrician should be called and the problem fixed before doing any work. If AC Voltage is not detected a Jumper cable bypassing the area you are working on provides an extra measure of Safety. These two things can save your life preventing a dangerous electrical shock.
Lets take a look at how the typical home electrical system works.
You should not ever have a ground that is carrying current.
If the ground is carrying current there is an electrical problem and an electrician needs to take care of it.
To the right is a diagram of a typical home electrical system which you may click on to enlarge. Yes, there are fewer breakers and the drawing was made easy, but, it’s the idea of how it works that I’m sharing with you.
It all starts at the pole… Or, the transformer box if there are underground utilities.
There is a transformer. It takes the power company supplied voltage and steps it down to 120 or 240 Volts depending on how your wiring is hooked up. The transformer has what is called a center tap, this is also the same thing as the neutral leg. There are 3 wires coming out of the transformer into the home. (2 hot leads and the neutral)
If you connect to either hot and the neutral you have 120 Volts. If you connect between the 2 hot leads you get 240 Volts.
The 240 volt loads that are connected from hot to hot are not really a concern in this discussion. What we are concerned with is the current that is carried on the neutral wire. Only 120 Volt circuits use this neutral wire.
The loads should be somewhat balanced on the 2 hot wires and the neutral carries only the imbalance of the load between the 2 sides. If the system is properly balanced the there should be a minimal current flow on the neutral in effect the major current flow ends up going from hot to hot through the balanced 120 volt loads. On panels where there is a severe imbalance is usually where problems may start appearing. Usually the problem starts as a simple bad connection on the neutral line. This bad connection has a high resistance. Current flow across this high resistance causes heat and burning of the wire and connector. As time goes on this connection becomes worse and worse.
Let’s look for a minute at imbalances on the circuits. I’m going to just throw in some numbers to make an example up.
Let’s say the circuit loads in the picture above are as follows:
#1-16 amperes, #3-10 amperes, # 5-10 amperes = 36 amperes total on the odd numbered breakers.
#2-18 amperes, #4-10 amperes, #6-6 amperes = 34 amperes total on the even breakers.
Breakers 7-10 are 240 loads which do not affect the neutral.
The imbalance between the odd and even breakers is 2 amperes so 34 amperes is flowing from hot to hot across the loads on the 2 different sides and 2 amperes is flowing on the neutral line. Life is good and this panel is nicely balanced, and the neutral is coasting through life.
Now lets go and look at a different panel leading a harder life.
#1-6 amperes, #3 -4 amperes, #5-2 amperes = 12 amperes total on the odd breakers.
#2-18 amperes, #4-20 amperes, #6-20 amperes = 58 amperes total on the even numbered breakers.
The imbalance between the 2 hot leads is 46 amperes so only 12 amperes is shared by the load and the neutral is carrying 46 amperes.
Both of these panels had the same total 120 volt load of 70 amperes but, it was distributed differently so one of the panels had 23 times the other in current flow on the neutral.
Now let’s look at the actual neutral and ground connections. The neutral is supposed to carry the current from the load imbalance and the ground shouldn’t carry any current. The ground is a safety circuit.
But, if you will notice, the neutral and ground are bonded in the breaker panel. When the bad connection on the neutral I was talking about earlier degrades enough that the ground becomes a more attractive path for current flow than the neutral with the bad connection the ground then becomes a current carrying part of the circuit. Electricity is lazy and always takes the path of the least resistance.
Now the hard to discern part, there should be a ground at the panel that connects to a tested ground rod copper coated steel driven into the ground. The NEC has evolved over the years and this was not always the case. At my mom’s house built in the fifties for instance there was a galvanized pipe that was driven into the ground. A couple of years ago while the lawn was being cut they bumped the ground rod with a push mower and it broke off just below the surface of the ground. We immediately replaced it of course but just how good was it providing a ground before it broke, and for how long. Some houses have the ground bonded to the metal water supply pipes. Even if they are not bonded visibly an electric water heater will provide a connection between the ground and the water pipes.
Pipes can very easily be carrying current without you even knowing it. When you cut the pipe and have one hand on each side of the cut pipe holding on to it guess who completes the circuit. The only safe way is to check the pipe with a Non-Contact Wide Range AC Voltage Detector Probe then after a safe indication, put a jumper wire connecting the pipe across the cut you are about to make. It could save your life!