Good afternoon I apologize for being somewhat vague because this is a government building and no cell phones and pictures are allowed kind of deal.I was asked along with my boss to look at a 125V D.C. system after the company i work for installed two small small systems.The head electrician at this site though our new systems had something to do with the problem to be had.So to the issue the building has a battery bank and two chargers running essential systems 52 cells at 2.5VDC.The head electrician there tests voltage on the bank monthly.He says the two chargers show a fault of "+ ground" on a warning light.He showed us how he checks it and it he checks from terminals to the bare bonding wire to the rack witch is welded to the floor and so on redundant bonding.Today he showed us 124v to- and 6v to + for a total of 130v 52*2.5=130.He then stated he normally gets 60 something on - and 60 something + :? and that is the way its been for years so we disabled our to systems and re checked the fault on the charger still stayed and had reads of 124v to- and 6v to + so anyway no fault seen in our additions but in my head I cant understand how or way the voltage would have ever been split 50/50 across the two or why the 124v to ground on the and six on the other.The only thing i can think of is there is voltage bleeding through some on the conduits and retuning the only way it can through metal floors pipes and racks?
Are there holes in my DC therory?
- Thread starter cad99
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DC theory is much like AC theory but without the complications of capacitive current flow.
If you have a deliberately ungrounded system, then the voltage from either line to ground will not be consistent and will not allow current to flow through a low resistance meter. The measured voltages to ground will not even necessarily add up to match the line to line measurement.
But if there is a large enough and uniform leakage resistance to ground at each battery, then you will end up able to read a balanced voltage to ground on the two lines. But you will still not be able to drive a significant load (such as a light bulb) from that voltage.
Your observation is consistent with a fairly low resistance connection or even a short between one of the battery cables and ground. Or else somewhere in the charger circuit.
Finding the defect may involve disconnecting the battery bank into its component parts. But you can localize you search by looking for the point along the battery string where the voltage to ground is zero.
If there is still a load connected when your system has been disconnected, then the ground fault could also be in one of the loads.
If you have a deliberately ungrounded system, then the voltage from either line to ground will not be consistent and will not allow current to flow through a low resistance meter. The measured voltages to ground will not even necessarily add up to match the line to line measurement.
But if there is a large enough and uniform leakage resistance to ground at each battery, then you will end up able to read a balanced voltage to ground on the two lines. But you will still not be able to drive a significant load (such as a light bulb) from that voltage.
Your observation is consistent with a fairly low resistance connection or even a short between one of the battery cables and ground. Or else somewhere in the charger circuit.
Finding the defect may involve disconnecting the battery bank into its component parts. But you can localize you search by looking for the point along the battery string where the voltage to ground is zero.
If there is still a load connected when your system has been disconnected, then the ground fault could also be in one of the loads.
zog
Senior Member
- Location
- Charlotte, NC
"Holes in my DC theory", that's funny
junkhound
Senior Member
- Location
- Renton, WA
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- EE, power electronics specialty
government building .....a battery bank and two chargers.... Minot or Grand Forks?, I know, can't tell or ya'd hav'ta shoot us?
Sounds like the battery bank for a UPS system like I've worked in similar places. May be a fault in the chargers.
If the chargers were made by a company in Denver (wild a$$ guess) I would check the fast-on (spade type) connectors onto the rectifier diodes in the charger. On one similar system I did troubleshooting on a few years back, just the magnetostriction noise of the charger transformers was enough to vibrate the fastons off a diode bridge, resulted in inbalance.
Sounds like the battery bank for a UPS system like I've worked in similar places. May be a fault in the chargers.
If the chargers were made by a company in Denver (wild a$$ guess) I would check the fast-on (spade type) connectors onto the rectifier diodes in the charger. On one similar system I did troubleshooting on a few years back, just the magnetostriction noise of the charger transformers was enough to vibrate the fastons off a diode bridge, resulted in inbalance.
Or conductor inductance.
But you are right. Unless the system is intentionally grounded (or earthed as we Brits would say), measurements to ground would be indeterminate, especially with a high impedance meter.
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