How to determine which is earth wire.

"It is common practice for this to be the negative but you can verify it by following that ridge to the cut-off alligator clamps to determine the polarity"

I was curious about that as I would have thought the marked wire would be positive so I went and dragged out the three items I have with 12V cords on them and on all three the positive wire was the one marked with the ridge so maybe not as common as you thought Alan.

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Well maybe I was wrong about that and your comprehensive survey has enlightened the engineering world. Or just maybe my extensive spread of experience provided a more comprehensive database. Did you consider that your three reviewed products possibly came out of the one Chinese factory?
But I give you credit for knowing how to flog a dead horse in order to score a worthless point.

Alan,

You are well know to criticise others for posting what you consider to be disinformation or a view point that differs from your own but it seems you don't like it when the same applies to you.

The three items in my garage, a 12v compressor, a 12V rattle gun and a 12V hydraulic jack were all made by different manufacturers and purchased over many years.

In my personnaly experience I have seldom seen the negative wire as the the marked one, generally it traced wire that is the positive.

As a matter of interest I Googled “what does the ridge on a two core cable represent”.
Try it Leigh.

Well lets see:

"The ridge or ribbing on a two-core cable indicates the neutral wire. The neutral wire is also identified by white insulation or a white stripe"

And

"The neutral wire is identified by a white stripe, ribbing or white insulation"

But then as you know we are talking extra low voltage DC here and not AC:

"Traditionally the line that is marked (whether with a colored/painted line or groove) is positive"
"Usually cable with white dashes means that that wire is VCC (i.e +12V)"

"the wire with the white stripe or the dashed lines carries the "positive" (+) end"

I have always found with DC the "indicator" has been the positive side this includes the units I mentioned above and not to mention the mirade of power pack units I have seen. But then others have indicated the ridge is negative on their units, so seems there is no common practice in the world of DC. Personnaly if I'm going to change a plug etc I always check before I chop of the old one off what the polarities are.

Yes, there are no universal standards on this and even if there were I would expect the Chinese to do it their way, whatever. Being no 12V industry standard it seemed logical for me to extend the LV industry standard and that seemed the American way. But I must say that they dedicate white for neutral and black for active in LV applications where the conductors have independent colours.

Malken has said that he realised his error too late but now has it by the ‘Short and curlies’. But I would like a Dollar for every time I forgot to slide the plug top cover onto the flex before terminating. I’ m afraid we are not all perfect. Although I can spell your name correctly. lol.

HOYKS.
The problem is: neither of the wires will be connected to the case of the compressor so the chance of shorts is eliminated from case contact.
I have 4 different 12v compressors and all are straight to motor after the switch; if present. No connections to case.

Leigh
Are you serious! ONE STROKE means the piston goes down and NEVER goes UP AGAIN. ALL compressors are TWO stroke. DOWN and UP is 2 strokes after all. My Whipper snipper relies on the concept to operate. A compressor must draw in air on a downward stroke. ALL compressors with pistons can pump air with their crank running in either direction. Lube supply critical!

RMD,

I believe all except you it appears understood what I meant when I wrote:

"they draw air in on the downward stroke and compress it on the upward stroke"

So they really are 2 stroke and not ONE stroke devices then? I see what you meant too, but it wasn't what you first said. Sort of cuts out being a "ONE STROKE DEVICE"!

You know you if spent as much time providing useful information as you do critiquing what others write this forum would be a much more helpful and friendly one and also have more members.

Bigfish
SO a ridge in a wire only ID's the SAME WIRE and not polarity.
Good point, NOT to ASSUME! On some items the Smoke can indicate the wrong polarity. Usually a definitive test!

Also Mikee5 was correct too, he took the time to check his same compressor and came to the same conclusion.

I checked mine yesterday out of interest. The ridge on mine is on the negative....As I said...do not assume as you have a 50/50 chance of being wrong. It shouldn,t affect the compressor but on most other 12 volt gear it could cause a melt down.

Allan.
I have previously seen comments after posts that the exchanges should be like discussions around a campfire. Yes, some departure from the OP's initial question may occur. Simply answering with a short reply may solve it for the poster, but often, other information revealed allows some learning to occur if folk are unsure. That way, peoples understanding and knowledge and ability might be increased. A positive thing. Is campfire talk to be ended? Nothing wrong with above replies or content , just some clarification too!

RMD,
I see completely where you are comming from and agree, but I also see where Allan is comming from.
Sitting round a camp fire with this lot that refuse to compromise or learn anything from other peoples experience would lead to someone getting stabbed.

Jarse, Malken says he is left with a pair of wires connected to his compressor with no apparent polarity markings. How is a multimeter, applied to those wires going to detect voltage?
Pretty simple, really?

Hi Allan, I was writing about the power source wires, as I misread the original post. If it is the wires are from the compressor, then some disassembly (of the compressor case) may be the only way to investigate further.

Jarse, if you had read Malken’s original post and my reply carefully you may have understood this as a very simple exercise which Malken has already acknowledged as solving his problem. The last thing Malken should do is to dismantle the compressor which is entirely unnecessary anyway.
No offence intended but these off-the-cuff responses only serve to confuse the enquirer.