transformer ground voltage

Discussion in 'General Electronics Chat' started by robb, Apr 16, 2006.

  1. robb

    Thread Starter New Member

    Apr 16, 2006
    4
    0
    1. I have a 12 volt doorbell tranformer and a 24 volt humidifier transformer. Both step down 120 volt line power. Both output AC. Both have terminal to terminal voltage on their secondaries (12v and 24v respectively), however the 12v transformer also has 12v line to ground voltage off one of its secondary wires. The 24v transformer has none. Why is this?

    2. In foreign countries using 240v only, how is power delivered into the house off the utility transformer? One hot 230v and 1 neutral wire?

    Thanks
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I am from "foreign country" so I can tell you. In central europe we have 1 hot wire 230V, 1 working ground and one safety ground. Current usually flows from the hot to working ground, safety ground should always be connected to metal chassis in case of some malfunction.
     
  3. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    1. Can you explain '....however the 12v transformer also has 12v line to ground voltage off one of its secondary wires'?

    2. I also live in a 'foreign' country. Here we have a 'live' 240v wire, a 'neutral'wire, and an 'Earth' or ground wire. There is no 'working ground' wire. Neutral is connected to ground at the service substation only, to prevent ground currents.
     
  4. robb

    Thread Starter New Member

    Apr 16, 2006
    4
    0
    Clarification on Question 1.

    The 12v transformer has two wires coming off its secondary. The voltage between these two is 12v measured with test meter. However one of these 2 wires
    also has 12v when connected directly to earth ground.
    The 24v transformer has two wires off its secondary with 24v between them, but neither wire has any voltage when connected directly to earth ground.

    I am thinking this has something to do with grounding on the secondary winding of the transformers. Is one grounded and the other not?
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    The 24v transformer is what I would expect.
    Regarding the 12v one. I have not come across a transformer before where the winding comes out to two terminals *and* one of them is connected to ground. It may be a faulty unit. Can you see a physical connection to the core? Is the 12v winding on a separate bobbin?
     
  6. robb

    Thread Starter New Member

    Apr 16, 2006
    4
    0
    The 12v transformer is very old, maybe it does have a fault on the secondary. I can not tell if primary and secondary windings are connected in any way. ( I would destroy unit getting inside it.)
    Incidently, I tested a third 24v tranformer, one that runs power to my furnace thermostat. It too has 24v line to earth ground voltage. It is fairly new. Could these transformers with ground voltage be autotransformers with directly connected primaries and secondaries?
     
  7. Gorgon

    Senior Member

    Aug 14, 2005
    113
    0
    Hi,
    Have you tried to measure with an ohm-meter if you have connection between the primary and secondary of your transformer? Have you measured the voltage between the live 120V and the 12v? Could the transformer be some kind of autotransformer?

    TOK ;)
     
  8. robb

    Thread Starter New Member

    Apr 16, 2006
    4
    0
    On 12v doorbell transformer, Ohm meter shows no continuity between primary and secondary. Voltage between 120v live side and one secondary wire is 12v. Voltage between 120v live side and other secondary wire is 27v. I assume this is not normal and some kind of fault current is in the device. If the general consensus is that low voltage transformers should not produce any line to earth ground voltage (only line to line), I'll take it at that.
    Thanks
     
  9. Grant

    Member

    Mar 5, 2006
    17
    0
    Hi Robb,
    Some transformers have one output of their secondary winding connected to ground, this ground is the iron core of the transformer (sometimes you see a spade connection for a cable lug on top of the iron core laminations) of which in turn is bolted to the metal casing it is housed in. With one side of the secondary winding grounded (a common), this can protect the winding/insulation or more importantly personnel from high voltage conditions that may occur during a fault, ie; short from primary across to secondary. This type of set-up is used in many industrial control and instrumentation (meter transformers) installations. You only need to "Google" a sentence with words; transformer, secondary and grounded to find numerous examples of applications.

    Hi Pebe,
    Firstly like to say I don't like the term "working earth/ground" either as I like to keep the ground and neutral terminology separate for safety sake.
    You say you have never seen the output of a transformer having one side giving a voltage to earth....previously you go on to nicely describe just the thing. Your residential power supply system is the secondary voltage of a transformer. One side of the winding is your active (or "hot" for the Americans) and the other side of the winding is a star point neutral (or wye connection) which is earthed in many places along the grid, sometimes also at your house. Earthed neutral- this system we call "multiple earthed neutral" (M.E.N) in my foreign country Australia.

    Cheers,
    Grant
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Hi Grant,
    Here’s a link to another forum that did a short article on earthing (grounding).

    http://www.epanorama.net/documents/groundl...rangements.html

    As far as I am aware the method of earthing called TH-S is the one used in most of urban UK. Neutral and earth for a circuit are connected only at one point – the substation – to prevent load currents flowing in the earth wire.

    When I mentioned earthed secondaries of transformers I was thinking of the ones you can hold in your hand that are used in domestic equipment – rather than the KVA monsters used for residential power supply systems.

    In UK power supply systems, the star point of the 3 phase secondaries – the neutral – is earthed to prevent them floating, giving possibly dangerous voltages relative to earth. I have not seen how it is done but I suspect that the earthing of the neutral wire is done at the start of the distribution cabling – rather than at the transformer.

    Coming back to Robb’s problem, I can understand that for convenience of wiring in a particular application, a transformer may be specially manufactured with one end of the secondary connected to the transformer chassis. But I would then expect only a single terminal for the other end of the winding and clearly marked as such, otherwise someone unfamiliar with it would see two terminals and assume that the winding was floating.
     
  11. Grant

    Member

    Mar 5, 2006
    17
    0
    Hi Pebe,
    I used the analogy with the high voltage residential distribution transformer to show that the reasons/principles are the same and also because you made made mention of this system! I think it makes interesting discussion anyway. One leg of the secondary output of the transformer is earthed for the same reasons small control/instrumentation transformers can be- to stop the high primary voltages appearing at the secondary in case of fault. Refering to the link you provided for the EU grounding arrangements, the TN-S system you say you have (supplier providing separate earth conductor back to substation), I would take substation as to mean the transformer. This substation being on the ground or up a pole! You mention you suspect earthing is done at "start of distribution cabling-rather than transformer" There is an earthing conductor running down a pole at each transformer. I see it as having to be at transformer secondary- as an earth can only be an earth if it is connected to the supply-the earthing cannot bypass many step down transformers and connect at the generator as this primary is isolated by transformer action so is a separate supply-have to earth at YOUR NEUTRAL.
    We have what the EU calls TN-C-S (protective Multiple Earthing), called M.E.N here in Australia- supply combines neutral and earth but they are separated out in the installation.
    With Robb's issue ( back to slightly smaller transformers!) sometimes a sticker is placed on the transformer stating "secondary earthed" and with a separate wire connecting to chassis. Maybe Robb's small doorbell transformer is done differently as something this small of this set-up is not normally dismantled and is sold assembled as part of a complete package, ........or maybe his unit just has a fault!!
    Cheers,
    Grant
     
  12. jw223

    New Member

    Jan 3, 2009
    9
    0
    Hi

    I think you need to check the 12v to line wire it might not be coming from your secondary winding, instead its possible that it is a provision for bonding your transformer non-conductive parts to earth in which case it wont have any connection with the 12v conductor. In any country 230v/240v is off the live and neutral of the 4-wire distribution system. At load centres the utility transformers make use of transformers with secondary star configuration and it is from this point that the 4 wire L1, L2, L3 and Neutral/earth systems originates and this is further reticulated in domestic and commercial consumers. I hope this helps.

    jw223
     
Loading...