kev.h replied:

Hi Vince
i have a two pin plug on my compressor (about the same size as a 240 v plug) and the positive red wire goes to the horizontal pin at the top of the socket and the black to the verticle pin at the bottom can only assume it is wired the conventional way done by an electrician
Hope this helps i guess it only matters when you plug into someone else's as long as yours are all the same way round should not be too much of a problem
Regards Kev

gottabjoaken replied:

Good question, Vince

My belief is that Aust Standard Wiring rules say as above, that the vertical of the T should be the Negative.

I also read that in Victoria (of course) plugs/sockets are wired the other way round. (because the horizontal looks like a minus sign - ummm.. oookay! sounds reasonable!)

The AS 2000 rules are only available for purchase, so contact a friendly (good) electrician who has a copy to confirm this.

Anyway, if you use anyone else's connection, CHECK IT!

When I do one for myself, I make the T vertical the negative.

but (please) lets have a qualified electrician confirm what the rules really are, with references!

Ken

joc45 replied:

The mobile radio communications industry has used these for the last 40 years or so with the negative being the horizontal and the positive being the vertical.
Easy way to remember is that the horizontal looks like a negative symbol, the vertical can only exist in a positive symbol.
The same standard is used for the white plastic 2-pin Utilux 15A automotive. connectors.
Gerry

Kiwi Kia replied:

Here are a couple of the references;

AS/NZS 3000:2000
4.9.1.3.1
Extra Low Voltage
Where an extra-low voltage electrical installation and an electrical installation of greater then extra-low voltage are in the same premisies, all socket-outlets supplied at extra-low voltage shall-

(a) have their voltage conspicuously marked;and
(b) be of a form that will prevent insertion of an extra-low voltage plug into a
socket-outlet connected to a circuit of greater then extra-low voltage.

Note: AS/NZS 3112 contains a specific plug and socket-outlet arrangement recomended for extra-low voltage applications.

4.9.1.3.2 SELV and PELV
Plugs and socket-outlets for SELV and PELV systems shall comply with clause
7.7.11.

7.7.11. Plugs and socket-outlets
Plugs and socket-outlets for SELV and PELV shall comply with the following:

(a) Plugs shall not be able to enter socket-outlets of other voltage systems.
(b) Socket-outlets shall not accept plugs of other voltage systems.
(c) Socket-outlets shall not have a contact for a protective earthing conductor.

From the above you can see that the answer to the original question (pin connection with respect to polarity) does not appear in AS/NZS 3000. As i have mentioned earlier the RECOMENDED layout is in AS/NZS 3112 which I do not have in front of me at present.

In the above quotes from AS/NZS 3000;

SELV means; Seperated extra-low voltage
PELV means; Protected extra-low voltage
Extra-Low Voltage means; Not exceeding 50 V a.c. or 120 V ripple-free d.c.

By using a battery or a stand-alone generator (the motor that drives the generator is not plugged into a mains supply) you are covered by the definitions SELV or PELV.

I will read AS/NZS 3112 and get the exact words for you.

tonysmc replied:

I have alway wired mine with the positive going to the top horizontal pin(as if its a T). The only reason I did this was that looking at a conventional 240 volt 3 pin plug has the earth on the vertical. I just thought it made sense to make the earth or negative on the 12 volt system on the same 'verticle' slot. Having said that not all wiring laws make sense, just look at the Australian standards for the wiring of 5 pin and 7 pin trailer plugs! Surely that was a joke that went wrong.
Tony

Andrew from TrekTable replied:

Here's a different way to view this topic that is always easy to remember.

Wherever possible, use the vertical pin as the negative because if the plug starts to work loose from the socket, the vertical pin will always maintain contact for the longest. ie The alignment of the horizontal pin (ie horizontally) makes it physically difficult for it to remain in contact after the vertical pin has lost contact. If you pull down slightly as you pull the plug out, the horizontal pin will lose contact before the vertical pin does. So if over time the plug starts to work loose due to gravity and knocks & bumps, the vertical pin will remain in contact.

Generally, you want the negative pin to maintain contact because it 'earths' the device. In higher voltage applications (eg 240V) it's very important for appliance to be earthed whenever power is connected (I won't go into the details around double insulation here, hence I use the word 'generally'). Hence, on 240V plugs, the negative is the vertical, to ensure the appliance remains earthed under almost all circumstances (especially if the plug starts to fall out over time).

Try it yourself. See if you can get the negative pin to lose contact first on 3 pin 240V or 2 pin 12V plugs. That's why the negative pin is always the vertical pin wherever possible. For safety reasons.

That's also why it's illegal to install 240V outlets in a position other than with the vertical pin at the bottom. To ensure the appliance remains earthed for until after the live pins have lost contact (especially if the plug starts to fall out unnoticed)

It only makes sense to apply the same logic to 12V systems.

Andrew

Kiwi Kia replied:

Sorry but I was not able to get a copy of the standard on Friday but I did ask a friend who works as an electrical engineer in the Min of Energy to check it out for me so hopefully Mon or Tue I should get a reply. The local tech training institute did not have this standard either ( I went through the teachers resource cupboards). I am sure we already know what the answer is but I WILL get the definitive words from the standard.

Kiwi Kia replied:

I have still not gotten my hands on the AS?NZS 3112 standard but try this site -

http://www.users.bigpond.com/lhoskin/T%20Plugs.htm

This is an Australian emergency radio group and they state the current usage of "T" connector plugs as used in Australia. Note, as someone else has already said, Victoria do it in reverse to everyone else !!

Also remember that if you are tapping into a vehicle battery system then you could have a (+) earth vehicle supplying the "T" socket in the reverse polarity to a (-) earth vehicle. TV aerials, magnetic base radio aerials etc. could cause problems if bolted to the metal work of your van, trailer, bus etc.

Also, :-) remember that some trucks, buses (house-bus / caravan) and 4wd vehicles are set up for 24 volts so even if the plug polarity is correct the voltage may be wrong !!!

I guess that the above explains why there is only a 'recomended' method of polarity connection for the 32 volt "T" plugs and sockets.

Ray H.