how do I ground a 25V circuit?

Discussion in 'General Electronics Chat' started by unsaint32, Mar 8, 2005.

  1. unsaint32

    Thread Starter Member

    Jan 13, 2004
    14
    0
    I have 277/480 Y at work and I need to hook up an exit sign that uses 25V. So, I am planning to use a transformer, I assume 277/25V transformer.
    I looked up the code book which said, I need to ground a system if it is less than 50V, and if it is fed by a supply system of more than 150V.
    But, how do I system ground the secondary side, which is the exit sign? Run a grounding wire to concrete? Even if I did that, what do I connect it to? All the electrodes, the building metal structure, and water pipes are being used as electrodes for the building.

    Am I wrong about the code?

    Jon


    What is the "objectionable current" at the code
     
  2. Erin G.

    Senior Member

    Mar 3, 2005
    167
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  3. AENEUMANN

    New Member

    Feb 15, 2007
    2
    0
    The way that most LED night lights are powered from 115VAC is to use a capacitor.
    For example, suppose the LED needs 10mA and the regulator circuit needs 10mA, or 20 Ma total.
    Calculate the size of capacitor which when connected to the 115V line and grounded to AC neutral & Earth ground combined (for safety) will draw 20mA RMS.
    The capacitor feeds a simple rectifier (could be a full wave bridge) to produce about 5V. If a single diode rectifier is used then the AC current must be doubled.
    The 5V regulator can be a simple zener diode. Connect the LED to the +5V supply with a limiting resistor such that +5V - 1.3V (LED drop) will result in the desired 10mA.

    This same method can be used to construct a very simple and potentially dangerous battery charger. Suppose you desire to charge up a typical auto battery at 5 Amps. Procure a 100uF motor-start capacitor rated for about 300V AC. connect this to a zip cord (AC cord) and then to a full-wave bridge. Out of the bridge comes a pulsating current of 5 Amps RMS. If the output is limited by say about 1 ohm and connected to the battery before the zip cord is plugged in then OK. However if the zip cord is plugged in first, then you have a 170V DC 5-Amp source that will electrocute just about anything. To make the thing safe, add to the output of the bridge rectifier na 1 ohm and then a power 20V Zener diode. This zener will limit the voltage to a safe level however still above what the battery needs.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804

    I don´t think that this is a type of idea to be presented to many unexperienced people. If the zener fails, your thought-to-be safe charger becomes deadly again.
     
  5. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Yes, you need to ground the step-down transformer because of the supply voltage level. More about that later. Your building power is known as a 480Y277 V, 3-phase system & 277 V is the voltage from any phase to your neutral or "Y" junction, which is grounded. Transformers with 277 V primaries aren't as common as those with 480 V or 240/480 V so you'll probably wind up with one of these. Ditto for 25V secondaries; 24 V is a common voltage & is near enough to 25 V to work fine. Also, make sure your tranny's big enough to supply all your lights if there's a chance you'll be powering more than one. Multiply the total current of of all the light fixtures by 1.25X25 [31.25] to get right size, in VA, then, if that size is non-standard, get the next larger one.

    As for grounding, the source for your 277 or 480 V is probably a grounded enclosure or junction box. Erin G. is right, get an enclosed tranny or put it in a metal box. Conduit will work as a grounding coundutor for your tranny enclosure as will your steel building framework & mounting hardware. Also, run a short jumper wire from a tranny mounting screw to a secondary terminal or lead; if the terminals or leads are marked, X2 is the customary grounded lead. Make this wire white to indicate that it's grounded & the grounding jumper green.

    I also recommend that you feed the transformer primary from a fused disconnect switch & it would be a good idea to fuse the ungrounded secondary lead to protect the tranny from shorts.
     
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