How to tell which side should be hot in older equipment - Page 1
Forum section: electronics / repair

author: dzarren
author_rank: Contributor
timestamp: April 09, 2018, 12:50:43 pm
content: im repairing a older scope that only has a two pronged plug. There is no bias for the plug in direction, both of the prongs are the same size. When wiring this back into the scope, which side should go to what in the device? The chassis is connected to ground, but not to either the hot or neutral.So I will put back the original cord, but i will mark it so i will always plug it in the same way.Or is the best plan of action to put on a new three pronged cord, such that no one will be able to plug it in improperly.I understand that it the device will work either way since its AC, but there is a better option between the two.So if i use a three prong, the ground would go to the chassis, but i still dont know how to determine where the hot goes or where the neutral goes.I would prefer to use the original cord, but am open to using a newer one if it means it will be safer.Here is a picture, which should the neutral and hot go to? This is what i am looking at.The twisted red wire goes to a fuse, and the other end is just the non fused side of the line cord, but I can still choose which side is hot or neutral. Speaking of which, is there a better side to fuse, hot or neutral?Is it even sensible to attach a ground wire to the chassis to mains earth it?

author: hermit
author_rank: Frequent Contributor
timestamp: April 09, 2018, 02:04:43 pm
content: Typically the fuse would be on the hot. Open neutrals can be problematic.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 09, 2018, 04:21:45 pm
content: IMHO it is almost always sensible to fit the gadget with a modern 3-wire mains cord.Possible exception would be a MODERN "double-insulated" gadget.The fuse is a prime clue about which side should be "hot" ("Line")I have a lifetime supply (~100) of the chassis-mount IEC 60320 C14 inlet connectors just for that purpose.I bought a bag of them at the Tektronix surplus outlet years ago.

author: dzarren
author_rank: Contributor
timestamp: April 10, 2018, 01:11:17 am
content: Quote from: hermit on April 09, 2018, 02:04:43 pmI was the one that placed the fuse in there, it did not come fused originally. So i have no idea if the fuse is on the hot. What do i need to look at to determine which side of the equipment should be connected to hot?Quote from: Richard Crowley on April 09, 2018, 04:21:45 pmOkay! I have a great number of those three prong connectors as well, i can certaintly use one and a three pronged cord.I'm just not sure what wire goes where. I figure the ground will go to the chassis, but have no idea how to tell which of the other two wires go where. Is there something I can probe in the circuit to figure out if one of the sides is meant to be hot?and again, the fuse was put in by me so it only has a 50% chance of being on the correct side, it doesnt give any information.I was also under the impression that the neutral side would also be connected to the chassis, but it is not.lots of components terminate at the chassis, lots of caps and stuff, but neither of the nodes that connect to the hot or neutral are connected to the chassis.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 10, 2018, 02:19:50 am
content: Quote from: dzarren on April 10, 2018, 01:11:17 amDescribe where each leg goes next. Is there a power switch? Do the two legs go into a power transformer or something?QuoteYes, the middle pin is the ground or "Protective Earth" (PE) That connects directly to the chassis.QuoteNO! Unless this is a very unusual scope (which you didn't identify?) it is very doubtful that it is "hot chassis".QuoteYes, follow the Live/Hot and Neutral/Cold sides all the way through to the (presumed) power transformer primary/input.QuoteYes, the chassis is the circuit ground, but that is AFTER the (presumed) power transformer.

author: james_s
author_rank: Super Contributor
timestamp: April 10, 2018, 03:28:28 am
content: Generally it's preferable to put hot on the side with the switch. Since it originally had a non-polarized plug it doesn't really matter.