Guide To Multimeter Safety and Input Protection

To say that most people don’t know what multimeters are would be an understatement. Worse yet, of those who do know what these devices are used for, not many are aware of just how dangerous multimeters can be if you use them in the wrong way.

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Consequences can be lethal.

A multimeter is a diagnostic tool used to gather basic readings on anything from circuit boards to AC mains. Knowing what applications are suitable for your multimeter of choice is essential if you want to avoid accidents that can damage your equipment, or much worse, inflict injuries to yourself. The first step is to realize that not all multimeters are created equal and that there are various versions of these devices designed for specific uses.

Our goal today is to brief you on multimeter safety. This includes discussing various types of multimeter voltage ratings, best uses and more.

With all that said, let’s start with the basics

1. Selecting the Right Multimeter For the Job

Most multimeters look similar if not the same to complete novices. Needless to say, this is far from being true. However, we aren’t talking about the actual type of multimeter. In other words, we aren’t interested in analog vs digital discussion. That is a whole different topic that deserves its own space.

No, we are interested in multimeter safety ratings. These safety ratings are generally related to two distinct types of incidents that can occur as you are using your device. One is arcing, which is pretty rare in home environments, while the other is transient overvoltage.

Transient overvoltage

Transient overvoltage is defined as a voltage surge that usually comes in the form of a very brief energy spike. These can be caused by a whole lot of things. Faulty equipment, lightning hitting transmission lines, or even the simple stuff such as turning on the power can produce transient overvoltage.

The hard truth of the matter is that encountering transient overvoltage is the cost of doing business. It is a hazard that is basically unavoidable if you are testing electrical equipment on the regular. Whether or not that transient overvoltage is going to harm you depends on the multimeter you are using.

One of the best ways to ensure your safety when using a multimeter is to choose one that has the proper CAT rating for the job you’re intending to perform.

2. CAT Rating

CAT rating is a standard that defines how high of a temporary energy spike your device is capable of enduring. As it turns out CAT ratings are very much location oriented. In other words, a higher CAT rating is going to be a must for applications that are close to the power source.

Here is the reasoning behind this. Say that you are working on a line that gets hit by a lightning strike. If you measuring anything with your multimeter 10 yards away from the strike, you will need the highest possible CAT rating, which is CAT IV in this case.

However, if you are working on equipment a few miles down the road, which is still hooked up to the same line that got hit by lightning, you might get away with a lower CAT such as CAT II or CAT III rated multimeter. It’s all about impedance that dampens the effect of a transient overcharge over distance.

Our scenario is based on a freak accident which you can’t really predict. However, what you can predict is the overvoltage that can occur on specific equipment, power mains and other environments where you might have to take multimeter readings.

This is where understanding the difference between CAT II, CAT III, and CAT IV standards is imperative, to say the least. You absolutely have to be prepared for the worst.

As things stand right now, these are the amp ratings for each of the categories we’ve discussed above:

– CAT II  – can deal with <10 kA

– CAT III – can deal with <50 kA

– CAT IV – can deal with >50 kA

Again, keep in mind that location is important as a CAT II device will probably survive a CAT III event that occurred 30 feet down the line.

3. CAT Categories and Voltage Ratings

What about voltage?

As it happens every CAT rating can be divided further into its own respective voltage brackets. That is why you will often see something like ‘CAT III-1000 V’ written on the back of a voltage tester. All of the CAT categories have these ratings that span from 400 V to 1000 V.

Keep in mind that CAT II – 1000 V is not going to be safer than a CAT III – 600 V as they are not in the same CAT segment. Once again, this is where you need to choose the right equipment for the job.

4. Input Protection

Input protection on modern multimeters can be quite impressive, but that’s not always the case. Just because a device comes with a CAT rating on the backside doesn’t necessarily mean that it actually features good input protection.

This is most often the case with affordable multimeters made by questionable brands. Without trying to burst anyone’s bubble, that awesome deal you’ve found on that no-name CAT IV multimeter is probably not a good idea. Sticking to reputable brands is the way to go here.

Back to input protection.

The first line of input protection fuses. Every multimeter is going to have them, there’s no question about it. However, what we want to know is the exact type of fuses used. The bare minimum for CAT II rated devices are the HRC fuses.

HRC Fuses

A regular fuse comes in the form of a piece of wire contained inside a glass tube. This design is old but still suitable for applications where overvoltage is highly unlikely. The problem arises when you put those simple glass fuses into high-risk environments.Let’s say that you’re measuring something and you get hit by transients. As that energy spike reaches your inputs, the fuse is the first thing to go. Depending on the rating of the fuse itself, it might go right away or it might take some time to heat up, but it will go nonetheless. Now, if you were using standard glass tube fuses, there is a chance that they blow violently and arc over. In other words, the glass is gone, the wire is gone, but the energy is still arcing over the and continuing to flow unrestricted.

With HRC fuses, that doesn’t happen. HRC Fuses are similar in design to standard glass tube units, with one important difference – they are packed with sand. This sand is there to prevent arcing and to absorb the energy even if the fuse blows on you.

Going Past Fuses

Fuses alone aren’t going to cut it either. Because of that, they are often paired with shunt resistors and diodes.

As we know, fuses can take time to heat up and blow. Depending on a variety of factors, this time can be short or long. The main problem here is that your shunt resistors might blow before the fuses do, which is something you absolutely don’t want to deal with.

What most manufacturers do is add a diode protection circuit that soaks in the extra current, thus giving the fuse enough time to blow. These diode arrangements can be quite complex, which is kinda the point. We are talking a number of diodes in a serial configuration. They can soak up a fair amount of voltage on their own due to the sheer number of diodes used.

Here’s the thing. Fuses blow all the time. Whether you have selected the wrong setting on the device, or you measure Volts on the Amp settings, chances are that you will blow a few fuses here or there. Having this diode setup at your disposal is a must. Any multimeter that doesn’t is honestly not worth using. Simple as that.

Voltage Jack Input Protection and MOVs

Everything we’ve discussed so far was related to amp lines. Voltage input protection is much simpler.

The standard setup is going to include a 3.5k resistor followed by a 1k thermistor which increases resistance as the temperature rises. The only issue with this type of input protection is that it doesn’t do anything for your regular, sudden transient overvoltage.

This is where we make a full circle back to CAT ratings and all that good stuff.

In order to have transient overvoltage protection, your PTC thermistor needs to be followed by a MOV. MOV stands for Metal Oxide Varistor. This is basically an open resistor that activates once the input current exceeds a set amount of volts. Once it shuts down, the thermistor has enough time to heat up and activate.

In essence, a thermistor is useless without one or more MOVs behind it and vice versa.

Here’s a question. Is it better to have one 1000V MOV or numerous smaller ones? The answer is a solid yes.

Multiple MOVs will ensure that there is no arcing and that you have much better energy dissipation. Because of that, you might find that some CAT IV-1000 V multimeters are capable of withstanding up to 1700 V transient overvoltage, thus technically offering much more than their nominal rating.

That is pretty much all that you should know about input protection and how it relates to CAT standards in general.

5. General Multimeter Safety

The last thing we want to touch upon is general multimeter safety. While CAT ratings and MOVs will undoubtedly protect you from high energy spikes, there are numerous steps you can take to minimize any risk of injury or worse.

– Always use a multimeter that is rated for your intended application – This can’t be emphasized enough. We are all human and we make mistakes, but if there is one mistake you don’t want to make, it is to use an insufficiently protected multimeter in high voltage environments. Check and double check your choice before taking any measurements.

– Always inspect your multimeter – One of the most basic safety rules with multimeters is to always assume they are faulty before use. This type of mindset will prompt you to inspect your equipment before use. What you are looking for is any kind of visual damage that indicates that the device has been dropped or used improperly. Next, you will want to check the multimeter by hooking it up to a known voltage source. This can be anything from a battery to a proving device designed specifically for this use.

– Always inspect the probes – Probes are usually the first component to go on multimeters. Because of that, it is imperative that you check them before every use. This means going over the entire component from the input jack side all the way to the probe itself. You are looking for damaged insulation, damaged probes and physical damage in general. If your probes turn out to be damaged, get rid of them. Using damaged probes is a huge no-no. You will need to replace them.

– Use PPE When Necessary – Last but not least, you should make sure to use Personal Protective Equipment when and where necessary. Taking a reading on your broken laptop at home isn’t going to require gloves and headwear. However, measuring stuff at your high voltage job site where you’re surrounded by high power equipment definitely will.

Conclusion

To summarize this short guide, you should always use multimeters rated for the job at hand. Double check and triple check this before working on any type of equipment. Furthermore, understanding input protection is essential. Don’t source your equipment from shady sources. If you’re suspecting that your multimeter isn’t meeting the CAT standard claimed by the manufacturer, you might want to use something else. A surefire way to check this is to look at the fuses and input protection on the voltage line. At the end of the day, staying safe is worth the extra cost.