Mains transformer isolation ratings - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: November 16, 2012, 11:20:56 am
content: Hello all.I’m building a piece of instrumentation which incorporates a series of independent, adjustable low-voltage power supplies. Each power supply has to be completely floating (not tied to earth). In other words, if there was ever a primary-secondary winding insulation breakdown in any of the power transformers there would be a potential shock risk to the user of the equipment.What manufacturing/testing standard(s) do the transformers have to comply with to be legally used with floating secondaries?

author: GK
author_rank: Super Contributor
timestamp: November 16, 2012, 10:48:49 pm
content: No one?As far as I can gather the transformer needs to be compliant to EN60950. However after nearly an hour of googling I still can't find a concise answer.

author: penfold
author_rank: Frequent Contributor
timestamp: November 16, 2012, 11:01:06 pm
content: It depends on the final application. Medical applications have very different requirements to some other applications, i believe EN 60601-1 (uk standard) is one of the main standards of that field.Chances are for your application you will be looking at field specific standards. The country in which the product is to be sold is also important

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2012, 09:55:33 pm
content: Standard mains transformers tend to have poor insulation between the secondaries (often <500V) so if you put mains on a circuit connected to once secondary, the other secondaries should be treated as though they're also at mains potential.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 18, 2012, 10:06:35 am
content: EN61558 handles a lot about transformers and is very commonly mentioned by transformer manufacturers. As some of the other repliers already mentioned, you must also check if there is a more application specific standard that applies to your product. At least in Europe we have a set of main, not mains, standards, application specific standards, products specific standards and also country specific amendments.A whirlpool bath, a Jacuzzi for instance, is a household product and therefor is regulated in EN60335. EN60335 mostly points out general details and which main standard that regulates that. For instance EN60664 for electrical insulation and EN61558 for transformers etc. On top of that you might find that the different standards can change the rules quickly with paragraphs like "Measurements according to section XX.YY.C in EN*****, but from the next higher voltage class in Table X" and "Measurements according to section XX.YY in EN****, but with an addition of X mm to compensate for manufacturing tolerances in mass production". Of course, most standards are cross referencing to each other, just to make life easier...Two recommendations:1. Take the time and check the standards first before you start designing the product. Are you allowed to use switching PSUs or do you need a "classic" transformer? Does EuP (power saving directive with standby current limits) apply to your product? What isolation voltage do you need for the transformer (4kV, 5kV)?2. Make several single supplies using the same basic design to speed up tests and simplify certification. Use one transformer for each output and, if possible, the same transformer type, manufacturer etc.

author: GK
author_rank: Super Contributor
timestamp: November 19, 2012, 11:56:41 am
content: This isn’t for a commercial product. But it doesn’t matter any more. I’m now designing my integrator banks along the lines Systron Donner used to pre-charge the initial condition(s) from a ground referenced voltage source, so my Initial Condition Reference supplies no longer need to be floating.I’m using a series of precision variable supplies in my +/-100V system design so that I can, between problem solving iterations, simply scale a selection of variables simultaneously with a single adjustment, instead of having to manually and tediously readjust each coefficient individually.I hope that clears everything up.BTW, can anyone recommend a cost-effective source of 100k 10-turn precision wirewound potentiometers, counting dials and bananna jacks/plugs? I'm going to need lots.