In this feature we look at measuring voltage, current and resistance – vital whether you’re trouble-shooting zone loops or installing and commissioning access control readers and strikes, or analogue CCTV systems.

FOR a start it’s important to have a grip on the fundamentals before you start measuring electrical properties during installation or troubleshooting. Remember that voltage is the electrical pressure that occurs between 2 points. It’s also called potential difference. In essence this potential difference is the result of one location having more electrons than another. 

An atom with more electrons than protons has a negative charge, while an atom with more protons than electrons has a positive charge. Electron theory is complex – there's still conjecture over why electrons have electric charges at all. Think of voltage as the pressure that makes electrons flow.

This flow of electrons is called current. It's measured in amperes (6.25 x 10 to the power of 18 electrons flowing past a point per second). In a basic circuit when voltage (a battery) is connected to a circuit, current will flow. Electrons will flow from the negative terminal down wiring to a light bulb, through the bulb generating light, then out of the bulb and into the positive terminal of the battery and around again. 

There's some confusion about the true nature of electrical current. Our definition of conventional current flow is exactly the opposite of the path electrons actually take – not very long ago no one knew electrons existed and by the time we'd worked it out, it was considered too late to go back and reverse the text books. Regardless, the 2 important things to realise are firstly that there is a flow of electrons and secondly that all equipment, circuit symbols and the rest, register what's known as conventional current travelling from positive to negative – so should you.

Now let's look at resistance. Think of a hose turned to full pressure. The water pressure is voltage and current is the water flow. Now put your foot hard on the hose, constricting it so that despite the original pressure (voltage) and water flow (current), only a small amount of water leaks from the nozzle. In this case your foot is resistance. In an electric circuit any opposition to the passage of electricity is called resistance, with the resistance of a circuit determined by its components and the resistance of its conductors.

Measuring up

Now we've got the fundamentals covered it's time to get our measuring tools out. Depending on how old your toolbox is this will either be an ammeter, voltmeter and ohmmeter, or a multimeter combining all these testing devices. 

Let's start with the easiest test of all, voltage. What's nice about voltage is that because it's always between 2 points in a functioning circuit, your voltmeter simply needs to be attached to the 2 locations voltage should be present. As a rule one of the points will be the circuit's common rail and you'll measure voltage from that point. A lot of alarm panels have the negative side of the source wired to the common rail, but it's not always the case. 

Of course in practice it's not an open and shut case. If you're measuring a voltage drop across the positive and negative terminals of a panel, or across any component that you believe may be the cause of a voltage drop, there are variations and vagaries depending on the problem and the overall system design. 

When testing you can think of a voltage drop as loss of pressure caused by too much constriction – too much resistance. You'll discover this point of higher resistance because the voltage will be higher upstream of the component than downstream. You'll calculate the voltage drop by subtracting the lesser voltage from the greater. The difference is the drop across the component. 

In the event that current isn't flowing any longer, things are going to be a bit more complicated. If, for example, the switch in an electrical circuit is open, testing either side of a component will reveal battery voltage. You can also voltage drop using Ohms Law, with the voltage drop equalling current x resistance. Putting your voltmeter across a component's resistor will allow you to measure any drop in voltage directly. Just remember to put the positive side of the voltmeter on the positive side of the resistor and the negative side downstream to ensure the voltmeter shows the proper polarity for a digital meter (upscale).

Current and resistance

When measuring current you actually need to get into the wiring by cutting or breaching the insulation of the wiring. In short, current must go into the ammeter at the positive lead and go out of the negative and in addition, the current leaving the ammeter must be identical to the current that went in – absolutely identical.

Resistance is measured with an ohmmeter – a unit that is essentially a meter complete with an inbuilt battery and circuitry. When you're using an ohmmeter remember that you need to ensure the resistor being measured has at least one end disconnected from the circuit and as the tester, you must only touch one resistor lead when taking a measurement. 

The reasons for all this are that should the resistor remain connected other parts of the circuit will impact on the reading you get from your ohmmeter, giving a bad reading, while a voltage in the circuit could damage the instrument. 

Another issue is that you, the tester, contain an electrical charge and can inadvertently connect yourself across the resistor. What this means is that the ohmmeter will measure the resistance of the circuit and of the installer's body. The reading from the body will be a parallel path resulting in a lower reading than would otherwise be the case.

Fact file:

Definitions to remember:

* Current is the flow of electrons 
* Voltage is pressure or potential difference
* Resistance is anything that opposes electron flow
* Ohm's Law states that Current equals Voltage x Resistance
* Atoms with more electrons than protons are negative
* Atoms with more protons than electrons are negative.


“In an electric circuit any opposition to the passage of electricity is called resistance, with the resistance of a circuit determined by its components and the resistance of its conductors”