When to use Isolation transformer? - Page 1
Forum section: electronics / beginners

author: primegaps
author_rank: Contributor
timestamp: April 02, 2018, 10:56:19 am
content: I think i understand why one needs an isolation transformer when working on old scopes/radios and such.But after I finish the restoration, shouldnt I ALWAYS need the transformer when operating the old scope?I'm doing a restoration on an older scope, and I will turn it on slowly using a variac plugged into an isolation transformer, which is plugged into the mains.I understand that the chassis can be hot on these older 2 pronged gear, so wouldn't I always need the isolation transformer even after the restoration and normal use, just in case it was plugged in such that the chassis was hot?I could replace the cord with a 3 pronged cord, and just not utilize the ground pin, and leave it disconnected, just so it cannot be plugged in the wrong way?How do I determine which is the right way?If i could i would like to keep the old plug, i do enjoy its aesthetic.so in summary, will i always need the isolation transformer even just for normal post restoration use? I dont see why not, but no one seems to do this and it is unreasonable? thank you.

author: edgelog
author_rank: Regular Contributor
timestamp: April 02, 2018, 11:13:00 am
content: The outside case of the scope isn't (I sincerely hope!) connected to the chassis, so even if the chassis is hot, the case shouldn't be. The case should be connected to earth.IOW, as long as you don't expose the internal chassis to touch or measurement (grounding through probes, for instance), you don't need an isolation transformer.

author: oldway
author_rank: Super Contributor
timestamp: April 02, 2018, 11:29:09 am
content: Your question is interesting and requires a rather complex answer because there are several points to consider.1) older appliances no longer meet current safety standards (Class of appliances)Either they must be totally insulated (double insulation or reinforced insulation = class II) or all accessible conductive parts must be earthed (class I)Old devices have conductive parts (chassis, metal case, for example) that are not earthed ..... What to do? Upgrade them? yes, it's better, but it's not always possible. The aesthetic aspect is a reason (object of collection).It is not necessary to use an isolation transformer.2) old radios: be careful, the chassis is sometimes connected to the mains ... As a precaution, always use an isolation transformer.3) switch mode power supplies: Always use an isolation transformer as a precaution.For older devices with a transformer with conductive parts that are not earthed, it is prudent to check the insulation between the primary and the metal chassis with a 1500V isolation tester.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: April 02, 2018, 11:55:01 am
content: The advantage of using the scope the way it is currently wired is you are not earth grounded and so less of a chance of causing damage to the front end. Since you have the scope apart why not determine the neutral side of the ground to chassis on the plug, mark it and put the plug in the outlet the correct way every time you use it?