Cat rating of test instruments 2

[TerryOL]

I am interested in learning more about the classification of test instruments, i.e. Categories (Cat rating). What I am specifically interested in is the level of fault current that determines the category of meter.

All of the information I have found uses a diagram showing power flow from a pole top transformer into a building then down to final sub-circuits.
Cat 4 covers the circuit from the transformer secondary to the building fuses;
Cat 3 the switchboard etc.
I do not have a problem with this description for a normal 300kva pole top transformer in the suburbs, however I have a couple of 2.5MVA 11/0.433kV transformers supplying an MCC, I expect my fault currents are a little higher than the examples generally given.
My question is: At what level of fault current (ka) do I put my Cat 3 multimeter back into my tool bag and pull out the cat 4 one?

Thank you for your help.

 

[itsmoked]

It's not about the fault current. It's about the magnitude of likely voltage spikes. Much smaller spikes are expected inside a building all the way down to an outlet, inside an appliance, etc., etc.

If you think you can have a large spike from something like a large motor being interrupted or a lightning stroke down the road a ways, then you need the higher categories. Your case sounds like it's outside the building where the lightning strokes roam freely. That means a CAT4.

Doesn't matter what the available fault current is, nothing out there will save you from the results. That's left to preventing an explosive flash-over in the instrument from ever occurring.

The instrument will have a properly rated fuse inside to limit a disaster...some... You're still boned if you make that voltage measurement with the meter probes in the amp holes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Yes, as Smoked says. A certain instrument supplier, which I have respected before, has turned the CAT thinking into a sales promoting tool and is publishing some material that I wouldn't trust too much. While they talk a lot about CAT, they almost don't mention prospective fault current at all and what it does to you.


Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

If you are connecting to a high energy source then use fused probes, don't rely on the meter fuse. Make the connections with it de-energised, then switch it in once they are connected. Switch it remotely if possible.

I was lucky enough to be present at some arc-fault withstand tests on switchgear at IPH in Berlin, and I promise that an 80kA / 300ms fault is plain scary. Just to put things in perspective, the initiating wire used in the tests was no larger than 2.5mm² cross section, which is about the size of most meter leads...


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If we learn from our mistakes I'm getting a great education!

 

[TerryOL]

Gentlemen,
Thank you for your input. I found the information I require in SAI Global publication HB178:2006: section in question below.
1.1.1 What is the highest category of electrical circuits you intend to measure?
Categories indicate the amount of energy available in the circuit. The higher the circuit energy, the higher the category required. The higher the category, the safer the meter.
CAT I—very low energy circuits e.g. Vehicles, battery-powered circuits (not connected to 240 V)
CAT II—low energy circuits e.g. Domestic use (up to 5 kA fault currents)
CAT III—medium energy circuits e.g. Industrial + commercial use (up to 25 kA fault currents)
CAT IV—high energy circuits e.g. Industrial + commercial use (above 25 kA fault currents).
From this and other information I understand the Cat # refers to the fault current (ka) the meter can safely withstand, while the voltage values refers to both the maximum phase to earth voltage the meter can safely measure and the peak transient voltages it can withstand. Therefore a Cat III 600 volt can be safely used on <25ka 600v to earth circuits with 6000v peak transients, while a Cat III, 1000v can handle 8000v transients.

 

[Skogsgurra]

Yes. Right.

So, you cannot rely solely on the instrument's CAT rating. You also need current limiting HRC fuses in your probes. And sometimes an isolation amplifier.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[stevenal]

The skinny little leads and traces on your CAT IV meter will not withstand 25 kA. As itsmoked said, it is about withstanding voltage spikes, not current. The CAT IV will withstand 8kV transients.
Available fault current can be used the gauge the magnitude and probability of transients, but the CAT testing is all about voltage withstand.

The meter of course should not be your only defense. You should be wearing the proper arc rated PPE in case that 25 kA fault does occur.