Page 229 - Boiler_Operators_Handbook,_Second_Edition

214 Boiler Operator’s Handbook

to open the circuit breakers in addition to disconnects sure until it exploded.
when servicing equipment so add them to your lock-out To ensure that can’t happen again all control cir-
tag-out procedures. cuits must have one leg grounded and all final devices
Maintaining grounds is a constant problem in (control relays and fuel safety shut-off valves) have one
many plants and I always rely on the eyes and skill of side connected to the grounded conductor. (A grounded
operators to spot problems before they become serious. conductor is a wire for carrying current that is connected
A common way to ensure a good electrical connection to ground at one point to ensure its electrical potential
between steel building structures and the ground is is the same as ground) Any ground that forms in the
installation of a grounding grid and bonding. A ground- control circuit should produce a fault that will trip the
ing grid is a pattern of copper rods laid out in the fuse or circuit breaker. If that doesn’t happen the ground
ground around and under a building to provide good should produce a short circuit between the fault and
contact with the earth, they are welded or mechanically ground so there is no voltage across the associated re-
attached to each other and to bonding jumpers that ex- lays or safety shut-off valves to keep them open.
tend to the building structure. Of course, if the conduit or other parts located
Bonding is the process of installing jumpers con- where the wiring insulation fails is not grounded it not
necting one piece of metal to another to ensure electri- only becomes a point of high potential that can cause
cal current can flow from one to the other. If buildings personnel injury. It’s also a conductor that can bypass
were not grounded lightning could create thousands of some of the limit switches on the boiler. To ensure there
volts of potential between the building and ground, let are no inadequately grounded metals around a boiler
alone the static electricity differences in a building from an annual check should be made of their resistance to
a cloud passing over it. If you touched the building with ground. Using a simple multi-meter set at the lowest re-
your feet in contact with the ground, well… you would sistance setting and one very long test lead check the re-
become the grounding conductor. sistance between the grounded conductor in the burner
Look around at the bases of steel columns and management panel and every metal object (except wir-
you’ll see an occasional wire run up through the con- ing) on and around the boiler. The resistance should be
crete to an attachment on the steel, that’s a bonding less than 5 ohms everywhere. Usually you will find the
jumper. The connections can be mechanical or the metals resistance is less than one ohm with 0.3 to 0.5 being com-
can be fused using a thermite welding process. Thermite mon. I chose 5 ohms because a little more resistance can
welding creates a puddle of hot molten metal that at- produce enough potential to keep a small control relay
taches itself to the steel and wiring. The bonding wires energized.
serve as the bonding jumpers because there’s no guar- Just like you check motors for overheating bear-
antee that the anchor bolts, nuts, and column bases will ings, you should check out your electrical panels and
maintain electrical continuity. switchgear for loose connections that generate heat.
The problem with those connections is they are The wiring can loosen especially when the equipment
exposed and can be broken loose by any number of is started and stopped frequently because the wire does
methods. Your effort should simply consist of noting heat up a little bit every time it runs and that results in
any damage to one and repairing it or having it repaired expansion and contraction of the metal that can loosen
immediately. Caution is advisable because there could the connections.
be a voltage difference between the two so always make Loose connections are very common with alumi-
certain you have no voltage difference before attempt- num wiring because aluminum has a larger coefficient
ing to restore a connection and be aware that any num- of expansion than copper. During a normal round you
ber of incidences in and around the facility could create just lay your hand on the front of each panel and com-
a difference, including a cloud passing over. pare what you feel to previous rounds. With large panels
I’m particularly concerned with grounds in and it’s a good idea to sweep your hand over the front to
around boiler systems because we’re dealing with so note hot spots which are indicators of loose connections.
much steel and water, all good conductors of electricity If you detect one plan to shut down that equipment to
(well water normally is) and lack of a ground invites correct the problem… before the equipment picks its
problems with control operation. The deadly explosion own time to go down!
of a boiler at the New York Telephone Company in 1963 Prior to annual inspections you should perform
was associated with ground paths bypassing some limit a detailed examination for hot spots at connections,
switches so the boiler continued to fire and build pres- opening panels whenever possible and scanning all