Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Floating Earth in a Generator

  1. Jul 19, 2015 #1
    I'm trying to find a correct circuit diagram for how a floating earth is correctly wired for a small generator that is not being bonded to ground using an earth spike. It may be that there's more than one legitimate way which is why I'm confused, but even manufacturer diagrams are contradictory - some showing a centre tap to the generator frame, others showing no centre tap; others showing an earth from the output socket back to the generator frame (but the frame not grounded) and variations of all three!

    Does anyone have a source they can point me towards that they are confident is accurate please!

    Many Thanks
  2. jcsd
  3. Jul 19, 2015 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

  4. Jul 19, 2015 #3
    Hi Jim

    Thanks for the link, but unfortunately the diagrams all relate to ground earth bonded systems; the coverage of floating earth systems is quite sparse.

    I'm trying to understand the specific case (maybe cases) where the generator is not earthed to the ground with an earth spike or similar. Thanks
  5. Jul 19, 2015 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

  6. Jul 19, 2015 #5
    Hi Jim

    This touches on floating earths again but not in and detail. I think my orignal question may be ambiguous.

    I'm not sure how to ask an admin / moderator to lock a thread, but I'll start by making the request here, and I'll start a new thread with a better constructed question - thanks for your help in getting my head straight!
  7. Jul 19, 2015 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    could be, why not just continue here ?

    a 110 volt generator wouldn't have a centertap

    as Mike Holt explained if the neutral is connected to the neutral of a structure it's not a separately derived system

    probably you are seeing manufacturers provide a degree of flexibility to comply with different installations.
    Can you post a drawing or two from generator manuals? That'd be a solid base to promote worthy discuccion.

    The basis document for earthing is IEEE standard 142, aka "Green Book"
  8. Jul 19, 2015 #7


    User Avatar

    Staff: Mentor

    Please just continue here in this thread. Multiple posting can clutter the forums.

    Can you say what your ultimate goal is? What is your application? :smile:
  9. Jul 20, 2015 #8
    Thanks gents. I'm specifically trying to understand the floating neutral of a generator that is not bonded to a grounded earth. I've seen multiple circuit diagrams some with a Centre tap and some without; and some with the neutral bonded to the generator frame but no suggestion that the frame is then earthed to ground.

    Are there multiple (safe) ways a generator that will not be earthed can be wired, or am I seeing diagrams with errors / diagrams that actually are meant to be earthed?. The Centre tap is particularly puzzling to me as I've only seen this before in 3 phase ground earthed diagrams.

    Last edited: Jul 20, 2015
  10. Jul 20, 2015 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    "understand "? meaning what ?
    Kirchoff tells us with his famous Current Law that current must get back to where it came from.
    He does not mention Ground or Earth.
    An entire system can float but it's usually not good practice.

    You're asking us to analyze diagrams that you haven't shown us..

    Here's another pretty good article with diagrams explaining what are the requirements and why .
    http://oshaprofessor.com/Portable Generators and OSHA Construction Standards 3-05.pdf

    In two wire single phase generators of less than 5kw it is okay to have both wires insulated from generator frame. That is, the white wire we call "neutral" need not be bonded to generator frame.

    In three wire (ie 120/240 volt) the neutral must be bonded.

    So yes, there is more than one way to acceptably build portable generators .
  11. Jul 20, 2015 #10
    Hello Mr Kill... Your post #8 touches on my concern. The PRIMARY reason for earthing is for safety. If you can not describe why you NEED the system to be isolated from ground, then you may not be fully understanding the outcome of building such a system. There are many ways to do what you are asking and they are almosy ALWAYS less safe than a grounded system.
  12. Jul 20, 2015 #11
    Thanks Jim. I'm on a phone at the moment and haven't been able to attach the diagrams, but will do so as soon as I get the opportunity to do so; thanks for the link.

    @windact. The safety element is exactly why small generators that are used to operate for example power tools are NOT bonded to a grounded earth. If they are earthed then there is a return route through your body in the same way as there is with a domestic supply. A floating earth means there's no potential difference between the tool and the earth that your standing on. In theory (and I'm sure it would be unwise to test!!) You could pick up either the out live or the return from a generator with a floating earth and not get electrocuted.

    The situation gets more complex when you have multiple devices on the circuit and that's when grounding normally comes in.
  13. Jul 20, 2015 #12
    Jim. Just had a look at your latest link. Even with the constraints of a phone it looks like it's going to answer my questions; it covers all the use cases I mentioned (including the center tap ) and has diagrams that make sense!

    I'll have a look tonight on a proper screen, but thanks in advance for the link. Most appreciated.
  14. Jul 20, 2015 #13
    All of the tools TODAY are double insulated - for this and other reasons. If you asking how to wire a permanently installed generator - without bonding - that is dangerous. The reason small portables are NOT required to be grounded is the risk of untrained people improperly making the grounding connection, or NOT making a ground when one is expected - is greater than the risk of the equipment insulation failure.
    In portable / temporary installations the risk is in the cables and connections - worn, or improperly connected, or wired backwards - etc It is expected that the user is not a qualified individual. In the US as soon as you take any generator an make it a permanently installed - it need to be properly grounded - the poor quality of the tool is not an excuse.
  15. Jul 20, 2015 #14
    @windact. Your views are fundamentally wrong and it's dangerous and inappropriate to peddle personal views like this. Have a read through the literature of any of the manufacturers web sites and you'll see that your views are at odds with their literature. Double insulation does not always protect from a fault within a device and manufacturer literature warns of the dangers of eating a generator that is designed to have a floating earth (just to pick up the two points you raised).

    My original request was not to wire a permanent generator as is plain to read, it was to understand the wiring associated with generators that have a floating earth. I've seen multiple (and I now know inaccurate) diagrams and I was attempting to correct my understanding.

    I respectfully suggest you look at the Honda web site (or any other manufacturer) where they cover basic generator safety. A floating earth protects a user better than a grounded earth in some circumstances and not as well in others. This post was to understand the circuit, I was not debating the pros and cons of each method.

    Kind regards
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook