What CAT for wall outlets?

[jtruc34]

This is a very simple question, but I found contradictory information. I'm not sure of it is CAT II or CAT III that should be used on mains outlets.

Thank you

[Psi]

https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

[jtruc34]

So CAT II should be sufficient?

[ArthurDent]

That question is answered pretty well with this link. It would help to know a little more about what the outlets you are going to measure.

https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

OOops, Psi beat me to it. Great minds think, etc....

[Psi]

That question is answered pretty well with this link. It would help to know a little more about what the outlets you are going to measure.

https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

OOops, Psi beat me to it. Great minds think, etc....

Great minds google it

[Ian.M]

If you read that carefully, a Cat II meter is permissible for working on outlets more than 10 metres from a Cat III source.   However if you use the wrong category meter and are unlucky^* the possible consequences are horrific, so IMHO it would be prudent to use a Cat III meter for *all* mains fixed wiring work downstream of the supply company fuses that are the boundary between Cat IV and Cat III. 

If a mains device doesn't plug in or connect via flex to a fused connection unit, don't use a Cat II meter on it!

* Its possible to not realise that an installation is not as it appears and has been left in a dangerous state. e.g: Is that outlet really 11 meters as the cable runs from your breaker panel or did the pot-growing previous tenants steal electricity from the neighbour's panel the other side of the wall less than a meter away?

[jtruc34]

Yes, I had already read that 10m rule. I find it a little strange. Why would it be less hazardous to work on distant outlet?

EDIT: apparently, it is because of the impedance of the wires, which increases with length, is it?

If a mains device doesn't plug in or connect via flex to a fused connection unit, [...]

What does that mean?

[Ian.M]

Wiring length directly correlates with resistance (given that the wire isn't grossly oversized) and therefore, (with the supply voltage) determines the maximum possible peak fault current if the circuit is shorted.  A fuse or circuit breaker has a rated peak breaking current.  If the peak fault current is higher, the fuse or breaker is likely to explode, and may continue to arc over and fail to break the circuit.  The fuse that explodes may be the one in your meter and if the Cat rating is fake or isn't high enough for the peak fault current, the meter may explode in your hand.

A fused connection unit is a 'Brit' thing - a wall box with a cord grip for flex, and a fuseholder for a BS1362 13A or lower fuse (same as used in British mains plugs).   In the context of meter categories, it means down stream of it, you are in the same risk zone as a plug-in appliance using a normal domestic outlet.   

[//Matt//]

Make sure that the CAT rating is tested independently or at least that the multimeter is manufactured by a trustworthy manufacturer. Don't probe mains with cheap Chinese multimeters. Some claim CAT III or II but don't have the needed features. If the CAT rating is legit CAT II should be good. 

[jtruc34]

It is not for buying something, it is more by curiosity. Mine is CAT-IV anyway, so I don't risk it to be underrated.

[Kleinstein]

Yes, I had already read that 10m rule. I find it a little strange. Why would it be less hazardous to work on distant outlet?

EDIT: apparently, it is because of the impedance of the wires, which increases with length, is it?

If a mains device doesn't plug in or connect via flex to a fused connection unit, [...]

What does that mean?

The distance to the distribution can make a difference in the peak currents in case of a short.
At university I saw the damage from a short at a outlet that was rather close to low impedance source:  the normal 16 A fuse (LS switch) did not handle the short circuit current well and 2 more higher current fuses at some 35 and 100 A blew. The cable (should have been 3x 1.5 mm²)  to the outlet turned slightly brown  - could be seen as it was on the wall and not inside.  In this case the outlet was maybe 3 m from a distribution/ fuse panel and that was right next door from the distribution transformer powering the rather large building.  So not a good idea to have such short cables ! We had luck that not much more happened.

[Neilm]

https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

CAT I was removed from the standard quite some time ago.

If your meter was made recently and claims IEC61010 compliance it should be rated CATIII minimum.

[bjbb]

Despite there's the 10m rule, most of the time a wall socket is considered CAT II regardless.

UL60950-1, which is the standard for ITE device power safety, considers all residential/commercial sockets CAT II, even if they are plugged right away from the panel.

I.E. they are all requested to withstand 2.5kV for household uses, even a CAT III 300V system can see potentially 4kV.

The basic reference for construction per OV category is the IEC60664-x series. Please note that most national standards based on IEC60950-1 are now essentially deprecated, and the correct reference for ITE and comm and A/V is now IEC62368-1, where most national versions are effective between now and 2020. National standards based on IEC61010-x and IEC60335-x remain the correct references for test equipment and appliances, respectively.

ANSI/UL 60950-1 does, in fact, both consider and allow and define conditions for designs per OV category I. But as overvoltage category I is for equipment having a mains connection where transient over-voltages are limited (that is,'protected' electronic circuits), you are correct that a design for connection to AC mains should consider at least cat II.

Finally, for North America,  Australia, Japan, EU, and others, the 'ultimate' and final deciding factor for ratings of construction and materials is the local/national building code (for example, NFPA70 for the U.S).