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Discussion Starter · #1 ·
I have a question about a ungrounded 130v DC system. why is it when you ground the + or - you loose voltage on one and you get 130v on the other?
 

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I have a question about a ungrounded 130v DC system. why is it when you ground the + or - you loose voltage on one and you get 130v on the other?
Because you are placing the ground at the same potential as the polarity you connected to ground.

With a 130 VDC system, From Positive of Cell one to the Negative of cell 60 you read 130 VDC. Now you take either the Pos of cell one or the Neg of cell 60 to ground. The ground is at the same potential as the cell you grounded..
 

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Because you are changing the system ground reference point.
Normally if you have ground detection you should have 65 volts to ground either side. If you have a ground then the ground becomes polarized making it the opposite side of the line.
You must also realize that you can have a high resistance ground fault. That becomes a voltage divider and the voltage to ground is divided across the fault resistance .
For example you could have 14 ohm 5 amp + ground fault. From + to ground you will have 70 volts and from ground to - side 60 volts for a total of 130 volts.

I have several 125 VDC ungrounded systems that I work on.

LC
 

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Discussion Starter · #5 ·
Ok Im starting to grasp ahold of it. Lone Crapshooter. do know of anywhere I get some good info on ungrounded systems?
 

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Discussion Starter · #7 ·
Ok cool. I haven't really work much around it so I'm trying to learn as much as I can. thanks for the help tho.
 

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Matt, there is a link in #6 post that I copied off the web at work.

The DC that I work with is operating power for large power circuit breakers.
 

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Discussion Starter · #10 ·
Im still new to using this site. where would I find #6 post?

bobelectric: I have been working with DC panels powering protections relay equipment.
 

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This is post 11. Post 6 is 5 ahead of this post between. Post 5 and post 7. LC
 

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Bilge Rat
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D C for what?
The 125DC systems I've worked with are for the control and relaying of medium and high voltage circuit breakers.

These breakers range from 4160 to 500,000 volts. They are a bit different than your basic 120/240 unit, but they accomplish pretty much the same functions.

The actual circuit breaker is nothing more than a switch. It can be operated by hand, or closed and tripped from a remote location, often by a PLC.

Since they are just a switch, relays are needed to trip them in the event of an undesirable condition. There are instantaneous overcurrent, long-term overcurrent, ground-fault and others.

Since these breakers will kill power downstream, in order to close back in, they need some sort of a power supply that is energized when downstream power is dead. This is usually accomplished with batteries.

The relays also need continuous power, so they are supplied with DC as well.

A typical installation will have one or more battery racks (usually totaling 125 volts, or thereabouts) and one or more chargers. The chargers will supply power during normal operation, and when their AC supply is cut off, the batteries will take over.

One really important reason why control power to the breaker must always be present is because if there's no power, the breaker relays cannot trip the breaker in the event of a fault.

Of course, this is a simple explanation that covers very little of a complete medium or high voltage installation, but you get the idea.
 

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Bilge Rat
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Forgot to add, most 125DC systems are ungrounded because this way either leg can have a ground fault and the system will stay energized.

Every one I've worked with has some sort of a ground-fault alarm that will alert someone if a ground fault occurs, but still allow the system to operate.
 

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Forgot to add, most 125DC systems are ungrounded because this way either leg can have a ground fault and the system will stay energized.

Every one I've worked with has some sort of a ground-fault alarm that will alert someone if a ground fault occurs, but still allow the system to operate.
Maybe a dumb question - but how does that work code-wise? I don't have a code book handy but I thought that at that voltage the DC- had to be grounded, and identified as such? Does the ground sensing device create an exception to this rule?

I've worked on lots of 125vdc as used for safety pull cords on long conveyor belts (like almost a mile long, so they have to be DC as there's so much cable capacitance that if you use AC the safety contractors don't switch off - haha). In those cases DC- was grounded.

Cheers!
 

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PPE Saves Fingers
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Maybe a dumb question - but how does that work code-wise? I don't have a code book handy but I thought that at that voltage the DC- had to be grounded, and identified as such? Does the ground sensing device create an exception to this rule?
I beleive that it comes down to 10-102(1) and the argument that it's supplying industrial equipment in a limited area, and has ground fault detection.

That, and if there's a problem, you really want things to work.

Tracing the ground fault is always fun. Blasted civil guys...
 

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The best part about split grounded DC is that you can read both the positive and negative to ground which really helps when troubleshooting. As far as the NEC goes it doesn't really seem to address how we use our DC systems. There are no fuses on the batteries because they would rather the batteries meltdown than lose DC power to the plant. On The DC powered emergency lube oil pumps the overloads alarm but don't trip the motor. They would rather have a motor burn out than destroy a multi million $$ turbine. All of the DC circuit breakers are way oversized as well so you have to be real careful.
 
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