This is a highly condensed training essay, typical of the no-frills subject matter KR Services concentrates into trainees. It was designed to be read daily while training.
Basic Troubleshooting Review by KR
There are two types of wiring diagrams:
The component arrangement diagram shows the physical arrangement of the components as they are arranged in the panel or unit. Wire colors may be shown also.
The circuit diagram (schematic) shows how wiring is connected between components, and how the internal electrical circuitry is arranged. The secret to using a schematic diagram is to simplify the diagram. When reading a diagram focus your attention only on that part of the diagram that involves the area you are testing.
The four items required for electron flow and to control it are:
1. a source of power such as a battery, commercial power, or transformer output, which must have a supply and return (common) side.
2. a path (wires) for electron flow FROM the source and BACK to the source. This is called continuity.
3. an electrical load such as a light bulb, coil or motor to dissipate the energy of the electron flow. A circuit without a load is called a short circuit and will overload the circuit conductors or source.
4. a means of control such as a switch, or a set of contacts in a relay or contactor. On a diagram contacts will be shown in the position that they are in when there is no power to the relay coil.
Keep in mind that some switches in relays are supposed to be open when the relay coil is not energized (normally open N\O) and some are closed (normally closed N\C)
The flow of current involves the continuous LOOP required to complete an electrical circuit.
The diagram shows the path of current flow from the power supply (source) through the lines (conductors) and through the switches and components (load) and back to the supply.
The power circuit is normally 115v, 240v single phase or 240/460v three phase.
The control circuit is normally 24 volt.
A schematic diagram is sometimes also called a ladder diagram.
Loads are normally connected in parallel, that is the same power line feeds them in a ladder type arrangement, The rungs of the ladder would each contain a load, and the rails of the ladder would be the two power conductors.
If the loads are connected in series, there would be more than one load on each rung of the ladder, and the voltage between the rails would be split between them. An example of this is a run capacitor in series with the start winding of a motor. Part of the energy is used in the capacitor, and keeps high current from flowing through the start winding. The other part of the energy is used in the winding.
When a switch is in series with a load however, it will stop and start the flow of power to that load by the opening and closing of it's contacts. If the switch is the only item in that circuit keeping power from flowing at that time, then VOLTAGE OR PRESSURE DIFFERENCE WILL BE PRESENT ACROSS THOSE CONTACTS. When the contacts are closed the voltage will be across the load instead of the contacts; causing electrical work to be done at the load.
If we know that a certain load is supposed to energized and it is not, then one of three problems exist:
1. there is no power and voltage at the circuit source.
2. a switching component or wire in the circuit is open or defective.
3. the load itself is defective
By checking the availability of voltage at each component on a single circuit of the diagram, we can find a switching component or wire in the circuit which is open. A common method is HOPSCOTCHING.
This is done by:
a. Connect one side of a voltmeter to the common side of the supply circuit. This side is normally available to all loads on one side in most circuits.
NOTE: THIS COMMON SIDE WOULD BE THE NEUTRAL LINE WHICH IS ELECTRICALLY GROUNDED ON 115V CIRCUITS, AND SOME 24V CIRCUITS.
HOWEVER, ON A 240/460V CIRCUIT IT WOULD BE THE L2 OR L3 HOT CONDUCTOR. IN THIS CASE MEASURING TO GROUND IS
U S E L E S S ! !
EXCEPT TO DETERMINE THAT THE LINE IS DANGEROUS TO TOUCH.
b. touch the second meter lead to the other side of the power supply to verify power availability on that entire circuit. If no reading is present then a fuse or disconnecting switch is likely bad.
c. work your way with this second meter lead toward the load; through the circuit through each component to the load; until you find a point at which you lose the correct voltage.
d. Whichever item shows voltage on one side , but not on the other side, is open.
Variations on this sequence are possible, but this is the basic way.
Measuring with leads across this item directly should show a
VOLTAGE (potential to do work) waiting to get across it.
Switches should never have a voltage across them when they are supposed to be closed. A load which has correct voltage ACROSS it's terminals (not to ground) is bad if it doesn't function. Keep in mind that some motors have more than two power feeds.
Current RESULTS from the voltage applied to a circuit.
An amprobe is very useful to troubleshoot a more complex load such as a compressor to determine if power is actually flowing through it's circuits. This is current measured in AMPS.
An ohmmeter can be used with the power OFF to determine the electrical conductivity in OHMS for verifying suspected problems within components. Also you can verify shorts, grounds and open components in circuits and conductors.
A SHORT is the lack of any resistance (OHMS) to flow (AMPS). No control at all on current blows fuses.
A GROUND is the lack of resistance to keep the current in the circuit from leaking out to ground. It too can cause a high flow out of the circuit, like a water leak on a pipe, and blow fuses. If one side of the circuit is already tied to ground, like a 115v setup, then you could call it a short to ground, since it is still bypassing the load like it isn't there.
An OPEN component has so much resistance to flow (Infinity Ohms) that no current can pass through it at all.
Always remember it only takes a few millionths of an amp of current to flow through your body to KILL you, be careful in any exposure to live circuits!