Page 51 - Boiler Operator’s Handbook

36 Boiler Operator’s Handbook

wiring connected to the ground. to the push-button in a conduit placed too low over a
Don’t confuse a ground wire with a grounded con- boiler melts and the wires touch each other (what we
ductor. Ground wires are there to bleed stray voltage to call a short)? c) a screwdriver somebody left in the mo-
ground, not to carry current. A grounded conductor is a tor control center dropped onto the terminal board for
wire that carries electrical current but is connected to a the starter shorting out that same push-button circuit?
ground wire. All the white wires in your house should d) Humidity in the electrical room promoted corrosion
be grounded conductors. If you took the cover off your on the metal core of the coil until the portion holding
circuit breaker panel you should see that they’re all con- the motor contacts rusted to it so the motor contacts stay
nected together in there and also connected to a wire closed even when there’s no power to the coil? e) two
that is attached to your water line (the ground wire). or more of the motor starter contacts fused together and
All the steel in a building, the boilers, pumps, pip- will not release even though the coil isn’t holding them
ing, etc., should all be connected to a ground. In cases shut? (I could go on with a lot of other scenarios) What
like the building steel or pumps and piping the electri- do you do? Make sure you know where to flip a circuit
cians will call them “bonded.” Bonding and grounding breaker or throw a disconnect in case something like
is the process of attaching everything that could carry that happens.
electrical current (but shouldn’t) to the ground below Keep in mind that disconnects are not normally
the building. At sometime in your career you should used to break circuits. They’re the devices that have cop-
have an opportunity to do what I’ve done, three times. per bars that are hinged at one end and slip between two
You’re working around a pump or something and step other pieces of copper that press against the bar to pro-
back or drop a tool and knock the grounding conductor duce a closed circuit. If you pull one of those to shut a
loose. There’s more in the section on maintenance that motor down expect some sparks. You wouldn’t normal-
addresses that. ly do it because those copper bars aren’t designed for
With everything connected to a ground the differ- arcing and they’ll melt a little wherever the arc forms.
ence in voltage between any wire and ground should When you do have to do it, do it as fast as possible.
indicate the voltage of the system the wire is in. System Speaking of arcs… you know, that spark between
voltages do vary though and you shouldn’t get excited if your finger and doorknob and the lightning are arcs:
the voltage seems a little off. The common 120 volt sys- they can be hazardous to you and the equipment. Ev-
tem will vary from a low of 98 to a high of 132 although ery motor starter and circuit breaker is fitted with an
they typically fall in the 115 to 120 range. 480 volt sys- “arc chute.” It’s constructed of insulating material and
tems usually range from 440 to 460 volts between leads designed to help break the arc that forms when you’re
at the motor. opening a circuit. You won’t see them used on common
We never give it much thought but you should al- 120 volt or lower circuitry because that’s not enough
ways know another location where you can disconnect voltage and seldom has enough current to produce a
the power to a circuit. Remember the toaster? The rea- sizable arc. Normally the arc chute has to be removed to
son you pulled the plug out of the wall was the toaster see, let alone get at, the main circuit contacts to inspect
control didn’t work. There’s usually a button or lever we and maintain them. You’ll recognize them after peeking
can push or flip to release the toast and turn the toaster into several starters and breaker cabinets. Whatever you
off but sometimes it gets jammed. That’s a regular for do, make certain it’s put back!
me because I like the whole grain large loaf bread and When somebody leaves the arc chutes off, and it
those slices are always getting stuck in the toaster. Well, happens frequently, the arc that forms when the con-
just like the toaster, you should be able to identify an- tacts open lasts longer and does serious damage to the
other means of shutting down every piece of electrical contacts because all the current in the arc tends to leave
equipment in the plant. through one point and that point gets so hot that the
Usually you just push a button labeled stop and metal melts and tries to follow the current producing
that’s all you have to do. The stop button moves a metal a high spot on the contacts. The next time the contacts
bar away from two contacts to open the control circuit close that high spot is the only place contact is made and
which stops current flowing through a coil that holds the metal is overheated because all the current for the
the motor starter contacts closed. The coil releases the motor has to go through that one little point. It melts and
motor starter contacts and the motor stops. The question the coil pressure pushes the contacts together squeezing
is, what do you do when a) the push-button contacts that melted part out until enough metal is touching on
don’t open? b) the insulation on the two wires leading the contact to reduce the heat. Then the contact is fused