Multimeter prob CAT

DinoDude

Member
I was helping a friend troubleshoot a problem with his headlights on his car. I asked for a meter and he gave me a CAT IV 1000V ebay special. I wouldn't trust that to keep my table from wobbling. And that got me to thinking.

I understand the importance of the CAT ratings and what they mean. If it doesn't say Fluke or HP/Agilent on it I'm pretty much not using it. There is just no way to know the internals.

Probes however are replaced somewhat often and I can understand not wanting to pay 50 $/£/€ for a new set each time. I've seen vast difference in quality. What makes each CAT of probes complaint? I know that 600V have long tips and 1000V have just a little sticking out. Is there anything else to it? What makes probes CAT II or III or IV. I would imagine insulation thickness has something to do with it but how do you check that? What do you look at when you pick up a meter that isn't yours? I'm thinking in an industrial environment. How do you know some accountant some where hasn't made a poor decision?

Thanks
 
Quite frankly, I've never given CAT ratings any thought. I buy a quality meter and use it. If there is bare wire or cracking rubber on the probes, I toss them. If the meter case is coming apart, I toss the meter. I use the right probe for the job. For most uses, CAT ratings are mostly irrelevant, other than knowing the manufacturer designed safety into the product. My attitude would probably be different were I regularly measuring voltages over 300 V, but that's a rare thing. What keeps you safe is using the thing between your ears, not the plastic on the probe.

In over 40 years of using multimeters, I have only experienced a major hazard once. It was an expensive, name brand meter that was several years old. It had an intermittent problem which caused it to sometimes read zero with voltage applied. That was hazardous because it indicated the power was off, when lethal voltages were still present. It was also embarrassing when I called the power company crew back out for a power outage that was really a broken generator transfer switch.
 
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I also never paid attention to CAT ratings, until we all started to become more aware of arc-flash hazards perhaps 20 years ago. The NFPA70E standard accelerated this change. I often found myself measuring equipment where there was over 50,000 amps of available fault current on the bus. Then I watched some videos (I think from Fluke) showing arc-fault accidents caused by crappy meters or people who set the selector switch to amps while measuring voltage. The results were dramatic and scary, and caused me to switch to CAT IV meters with integral current-liming fuses. The cost premium seemed minimal for the reduction in risk.

I often say "Based on the electrical stuff we did back in the 70's and 80's, by all rights we should be dead." Using a CAT IV meter is no substitute for working smart, but it sure eliminates one major source of arc-flash problems.

Also: "There are old electricians and bold electricians, but there are no old, bold electricians."

My 2 cents from the "old guy" seats. :)

ST
 
Steve I glad for your input. Things like CAT ratings are mostly irrelevant or I'm only measuring 120/204/220V is dangerous thinking. How many volts are you measuring when that 20 ton HVAC unit turns off. Inductive kicks can be huge. An arc-flash is bad news.

A lot of folks don't realize exactly how may amps are available in those dimmer racks. My small venue "only" has 125 amp 3 phase service. I don't know the curves of those breakers but they aren't instant. The actual available overload current is huge. We have IEC309 connectors in several places on our stage. That's three phase of 240V at 32 amps, the pins are big enough to stick a finger in, but I've seen people act like it's just another extension cord like they are putting up Christmas lights.

I do agree with the sentiment of buy quality equipment, replace it when it's worn out with quality equipment from a know supplier, and working smart. Leave the big issues to the sparkies. When it comes to friend, coworker, or company gear I'm just going to keep trusting my gut and grab my own when in doubt.

They say the same things about pilots as well.

Danke
 
We have 2000A at 208V 3-phase. I'm comfortable around the gear we use to distribute, including the 600A switch for our racks, but only because i have A) healthy respect for the danger and B) the knowledge of when a licensed electrician is needed.

Also, interesting note is that most fatalities in the US are through 120V systems. My guess is homeowners or handypersons underestimating the danger.
 
Back in the day, I loaned my Fluke 77 to an electrical engineer. I don't know the specifics, but believe he tried to measure the 26KV on the primary of one of the arc furnace transformers. It gave funny readings after that, I think due to many of the PCB traces being plated onto the inside of the case. I got a new 77 out of the deal and immediately traded it in on the then new 87. Which I have and which still works fine but since it's not CAT IV, I keep it at home for 120 and down. At work I use either a Fluke T5-1000 or a Fluke T+ Pro, both of which are CAT IV.

We are required to carry rubber and leather gloves, all test equipment must be CAT IV, we wear arc flash resistant coveralls, all cotton clothing and EH safety boots. Above a certain rating, we don the moon suit which is a right royal pain.

Michael
 
Use the 87 with caution. That's the model that I mentioned reading AC voltage intermittently. They were nice meters in their day.
 
Use the 87 with caution. That's the model that I mentioned reading AC voltage intermittently. They were nice meters in their day.

My 87 is over 32 years old and while it has never shown zero volts where there is power, it does show AC voltage that's not really there. I call this phantom voltage, it's caused by energy being coupled from wires carrying current in close proximity to the circuit being measured and only shows up if the circuit being measured is open at one or both ends. In these cases, you need a low Z meter or a meter with a low input impedance that will load the circuit a display the correct voltage. That's what the Fluke T+ Pro is for, it's a modern wiggy that is CAT IV rated. We have hundreds of PLC outputs that are solid state and at 120VAC. The leakage current is enough to trick a high impedance meter into displaying 120VAC when the output is offf and can be enough to give you a tingle, the T+ Pro shines here too. The other issue I have with the 87 is that the test leads seem to develop an oxide layer that makes low resistance readings off be several ohms. I wonder if your lying meter had the same issue? I regularly clean my 87's probes tips with a mild abrasive like 400 grit sandpaper or Scotch Brite. Now that I type this, I'm thinking this could be why your meter lied to you. A new set of leads may fix it.

Michael
 
No, it was an internal problem. Removing the plating and leaving a film of silica on probes doesn't seem like a good idea. A spritz of Caig DeoxIT would be better. Scotch Brite does no harm.
 
Years of abusing the probes at the steel mill probably wasn't good for them either. They are a bit shorter and less pointy than they were new. I'm thinking I'll get a new set although the 87 is in semi retirement due to it's not being CAT rated at all. I mostly use it for vehicle trouble shooting due to it's fast response time. At my bench I use either the 77 I found at work and which is in near perfect condition and for some reason has CAT III leads or the M9803R from Sinometer which is a cheap Chinese bench meter that runs off a 9V or a set of AA batteries or AC power. It tends to show DC voltage without being connected in circuit as many Chinese meters do but settles right down when actually being used. It has CAT III leads alsobut I don't trust them nor do I trust the meter to actually be CAT III rated.

Michael
 

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