Checking Wires

Checking Wires:
Faults in electrical systems may be due to faults in electrical wires and connections. These faults can be due to interference from other electrical systems, mechanical or chemical effects.

Check Connections Visually:





For all readings and checks where contacts have to be undone, check connections visually.
Checking:
- Look for oxidations which may affect contacts.
- Check that connecting pin is complete and that connector wire is well secured to the pin.

Open Circuits:











Open circuits show up when something fails to work. Worn cables and connections which have come loose can be common causes.
Checking:
- Undo connections at both ends of the cable. Take reading with ohmmeter across the ends of the cable. The ohmmeter reading should be about 0 ohms if there is no break in the cable.
- In some cases, it may be difficult to get the cable loose.
- If one end of the cable is connected to a GND point, you can put the test probe directly on the GND point.
- If one end of the cable is connected to a branching point, you can push a test probe with an extra-fine point through the cable sheath. Then take reading with test probe inserted on both sides of the branching point. (If the hole in the sheath gets too large, cover with insulating tape.)

Short Circuiting To Ground (GND):








A short between a live cable in a circuit and GND often shows up by the fuse blowing when power is applied to the cable. Check:
- Activate all circuit-breakers, sensors and switches on the circuit and check if the fuse blows.
- Shake cables and pull connections gently while taking reading to find any intermittent fault.
- Undo component connections in the circuit so they do not affect readings.
- Use ohmmeter to measure between wires and GND to find any GND shorts. The ohmmeter reading should stay constant with no components connected.

Short Circuiting Across Cables:





Short circuiting across cables do not always cause the fuse to blow. These faults usually show up when a component malfunctions.
Check:
Shake cables and pull on connections gently to find any intermittent fault as well.
Reading 1.
- Take readings with voltmeter at different points on the circuit while operating circuit-breakers, sensors and switches.
- The voltmeter reading depends on the circuit concerned and circuit-breaker and switch settings. Use circuit diagrams and signal tables to find the right voltage for the circuit.
Reading 2.
- Use ohmmeter to take readings across suspect cables to see if there is a short between them.
- The ohmmeter reading should not vary when reading across wires not connected to one another in circuit.

Resistance Of Contacts And Connections:








- Resistance of contacts, cables and connections should in fact be 0 ohms: but there can be a certain amount of resistance due to poor wire connections, oxidation on connections and wear. Too high a resistance can cause malfunctions: the point at which this happens varies, depending on circuit load. The usual value is a few ohms. Check:
- Shake wires and pull connections gently to see if there are any intermittent faults as well.
- Test with a loose female pin to see if male pin gives sufficient contact and does not come loose if you pull gently on the female pin.
- Use an ohmmeter to take reading across ends of wire.
- Take readings with circuit-breakers, sensors and switches connected to see if there is any excessive contact resistance there.
- The ohmmeter reading should not be more than a few ohms in a circuit with acceptable resistance in contacts and connections.