Actions
Forum Thread Details
  • State Suggested Answer
  • Replies 37 replies
  • Answers 2 answers
  • Subscribers 364 subscribers
  • Views 5104 views
  • Users 0 members are here
Related

Are there any adverse effects from isolating the AC power supply on my electronics bench?

4ringfan

happened to show me an Ebay listing the other day and it was like feeding candy to a baby.  My dad, he can be such an enabler!  Hehe!  So, the long and short of it is that I have acquired this BEAST (a Powervar 12 Power conditioner) and I am planning to modify it so that I can isolate and supply power to my entire bench.  The Powervac 12 is capable of supplying 12Amps at 120VAC, so the only thing I was thinking that I needed to be concerned with was possibly exceeding the supply capabilities of the transformer -- however I find the possibility of drawing that much power quite remote. So...


Here are my primary questions:


  1. Is there any reason or side effect that would contradict hooking the entire bench on the isolated secondary side of a transformer?
  2. Are there other things that I should be concerned with safety wise or otherwise plan for?
  3. Modification suggestions?
  4. In general, is this a good or bad idea?


I haven't found too many articles that talk about this, but here are links to a few that I have looked at:


isolation transformer - Electronics Forums


RadiolaGuy.com : Sonny's Tech Tips


http://www.signaltransformer.com/sites/all/pdf/IsolationTransformers%20Increase%20Safety%20of%20Electronic%20Systems.pdf


isolation - Why are we not always isolating the mains supply? - Electrical Engineering Stack Exchange


I have read the following and I take it that I should not connect any of my test equipment to the isolation, only projects that I am working on. What about power supplies?  What is the take on this?:

 

"DO NOT plug your test equipment into the isolation transformer, just the radio. I've seen folks wire their isolation transformer to a plug strip with all their equipment plugged into it and wonder why their "scope's" ground lead went up in smoke when they connected it to the radio chassis. If all your equipment is plugged into the isolation transformer, then nothing is "isolated"! Only the radio (or television) under test should be plugged into the isolation transformer!"


image

image

image


image



  • 0 in reply to clem57

    If you're holding live cables with a CSA that large.... yes.

  • 0 in reply to 4ringfan

    Ignoring the Breaker for a moment

     

    simple electronics

     

    if you want a current to flow, it MUST have a return path, there will be a positive and a negative and current will flow from one to the other

     

    If this is completely isolated as with the transformer or just take a 240V battery image (Without grounding) then how will you get 5mA or any leakage for that matter if there is no return path other than the two wires directly connected to the source.

     

    going back to the breaker now

     

    even though it is not implicitly connecting a ground to anything, both the power wires run through the sense windings (Keeping them in balance). if this is the only possible paths between the source then it can NEVER be unbalanced.

     

    It will become unbalanced if an alternate return path is provided that bypasses the common sense coil in the breaker, this is the return path typically via ground or some other wire that may not be part of the main circuit and if say a human touches one wire and ground it will trip the breaker

     

    if your not grounded and you touch both wires flowing through the coil of the breaker and there is no alternate path, you will simply electrocute yourself and the breaker will happily let you as all the current is still flowing through the breaker sense coil keeping it in perfect balance.

     

    For the breaker to trip there MUST be an alternate path back to the source that bypasses one of the wires in the breaker

     

    the Ground ensures this will happen by providing a low impedance path back to the source

     

    Every domestic household mains wiring example I found searching Google had some form of ground connected to the neutral wire either at the electric company transformer or at the house and in some cases both. The example I showed above included US/Canada and the UK, the US/Can working on 110V ish and a 2 phase system, the UK based on three phase with the centre of the star connected to ground

     

    Of course there are other variants but they will all have the same basic concept... provide a reliable return path that is not expected to conduct electricity under normal operation that in a fault condition (Human short or mechanical / electrical failure) will cause an imbalance in the circuit and trip the breaker

  • 0 in reply to Robert Peter Oakes

    simple electronics

     

    if you want a current to flow, it MUST have a return path, there will be a positive and a negative and current will flow from one to the other

     

    If this is completely isolated as with the transformer or just take a 240V battery (Without grounding) then how will you get 5mA or any leakage for that matter if there is no return path other than the two wires directly connected to the source.

     

    Oh, man, face palm! Thanks, Peter!  LOL!  I was looking at this thinking that the GFCI would work and that a leakage to ground on either secondary lead would produce a current difference and the GFCI would trip out.  I was not even thinking about the "5mA leak" needing to make the ground loop and that with an isolation transformer with no grounding on the secondary, that there essentially is no return.  The leak circuit would be open, so there should be no leak, so the GFI shouldn't trip.  If the secondary wires were truly perfectly isolated and there isn't a ground loop, I should also be able to grab any individual wire and not be affected though either, correct?

    if your not grounded and you touch both wires flowing through the coil of the breaker and there is no alternate path, you will simply electrocute yourself and the breaker will happily let you as all the current is still flowing through the breaker sense coil keeping it in perfect balance.

    Grabbing both wires obviously just makes you the load, a crispy fried load, but a load none-the-less.

     

    Oh, and in case anyone asks, I'm still alive because I ask questions. image

  • 0 in reply to 4ringfan

    Your'e still alive because you take advice, and tales of woe (experience). image

  • 0 in reply to 4ringfan

    Sorry I took so long to jump in but I've been very busy the last few weeks, at least I got there before your cooked yourself image

     

    I think there has been a few situations recently where members have been lets say not 100% on the implications of safety so I try to take these subjects seriously and may not weigh in unless I feel there is someone's safety at risk, a bit Like Mark and co re a certain mains relay board. This is by no means in the same category but I felt I had to say something based on my understanding of the solution

     

    If I got something wrong here guys, please let me know, I have as much to learn as the rest of us and you have all been kinda quiet since I responded ?

  • 0 in reply to Robert Peter Oakes

    you have all been kinda quiet since I responded

    I think you simply filled in the last part .... we had earlier discussed using mixed isolated/non-isolated and eventually got the plan 'more refined'.

     

    Mike is still playing with XRay machines and if we add microwaves he might have the opportunity to 'cook' himself. image


    Mark

  • 0 in reply to mcb1

    okay, all I was saying was that because the secondary live and neutral are isolated you can connect secondary neutral to earth and achieve a situation that does not exist in a normal building supply. Building neutral and earth are connected somewhere but there is always some potential between neutral and earth in your test equipment and UUT. But this set up allows you to eliminate that which means you can put a scope probe earth on neutral with any damage.