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125V DC Power

125V DC Power

125V DC Power

I'm an electrical designer and I just want get an insight on some of the more experience electrical engineers or field personnel with regards to 125V DC Power supply. We are installing a digital generator watt hour meter and we want to be able to monitor the values before the generating station goes down (unit offline). The current meter that we have is an electromechanical type and the operators are able to read it even if the unit is offline and that is not the case with the digital display hence we need a 125V power source.

I have found a 125V DC distribution panel that I can tie the new wattmeter into for auxiliary power. (picture attached)

What are some of the considerations that I need to take into account when tying into one of the circuits?

Some of the things that I have considered are the distance to the watt hour meter and also if there is a high likelihood that someone is going to turn off the breaker if the equipment downstream is being serviced cutting off the auxiliary power to the meter and the operators wont be able to read it. The operators use the readings to calculate the fuel consumption for the unit.

I'm thinking of tying it with the protective relay panel or the auxiliary cabinet or something that is not a pump.

Your input is very much appreciated as always. Thank you.

RE: 125V DC Power

Howdy H2L,
The 125Vdc panelboard is the logical place to obtain 125VDC for your power meter. The other possible locations might be a sub-feed from the relay-protection panel or the aux panel.
I am sure that you will find that all of the critical systems will be powered from this 125VDC panelboard.
Most modern digital power meters only require a few VA (or W) or power to operate, so voltage-drop won't be an issue.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: 125V DC Power

Keep in mind, some facilities keep the DC (battery bank) and AC isolated from one another, including the ground. Make sure this is not one of those facilities (typical for utility power generating facilities using battery banks).

RE: 125V DC Power

Thanks GroovyGuy and buzzp. These are all great learning points. I tried to trace all the wiring to the other circuits and it seems that there are spare terminal blocks that can be used to get power from.

Buzzp, can you please elaborate on the battery bank and the AC?

From my experience we try to not put the AC and DC in the same terminal block. So I have to double check to see if they share a common ground?

RE: 125V DC Power

Correct me if I'm wrong, but most of the 125VDC backup systems I've seen in power plants are not grounded, other than through the Ground Detection Lights (if any).

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: 125V DC Power

Howdy Jraef,
You are correct, all 125Vdc system are normally ungrounded and usually provided with some means of ground detection (ie either lights or some wizardry in the battery rectifier).

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: 125V DC Power

Yes, no ground (earthing) on 125VDC. I believe this is also an NEC requirement due to the voltage of the battery bank.
If they want to see this meter at any time, regardless of equipment status, you will have to power it direct from the source via a breaker or other. Powering it from protective relays isn't ideal as it adds another possibility of losing your primary protection should the WHr meter fail/short unless you fuse it separately. Even then I would only do this as a last resort. Those relays are also taken down for testing on a periodic basis. I would look for a DC panel with a spare breaker. That is the ideal solution. You may also find some WHr meter with a battery backup. Display off without normal source of power and comes on when press button or something - I don't know if this exists but worth checking into it. Good luck

RE: 125V DC Power

Really a spare breaker? This device will use very little power, and I think the smallest fuse you can find is about 3 amps.

But if you have loads of spare breakers, then you can do that.

RE: 125V DC Power

Most panels are designed for future expansion. The easiest is to use a spare breaker if you have one (15A or less so don't have to run bigger conductors). Why not use a breaker? Yes it is overkill but I guarantee it will be the most economical compared to trying to figure out where to pull power from, make sure it will be powered up when they want to read the meter, wire up a fuse, etc.
Hmmm, there are also many fuses available under 3A rated 125VDC.

RE: 125V DC Power

You may not want to power down the relays just so a metering technician can change out the wattmeter, so you may want a disconnect/breaker/fuses upstream if you power it from a protection circuit. We place 5 amp fused disconnects adjacent to most SCADA and metering equipment to provide short circuit protection and to allow for easy isolation.

The fuse is only providing short circuit protection, so standardizing on a 3 A to 5 A is OK regardless of the VA load of the meter. The fuse just has to be small enough to coordinate with any upstream device feeding a protection circuit. Going too small on the fuse may lead to fuses blowing when turning first turning equipment on.

In general, before adding load to a DC circuit thought should be given to the energy capacity and runtime of the battery as well as to the load on the individual circuit. Adding a couple of watts on every panel in a substation can end up with a noticeable impact on battery capacity. In this case since there a 225A pump on the DC panel, the batteries are probably pretty big.

RE: 125V DC Power

In our case most of the meters are on the same DC breaker, and each with it's own fuse. But given that we have several on the same circuit it works for us. There is also typically two DC panels, so we would have plenty of Dc breaker locations.

But my point is if you are not very careful, you can fill up a DC panel and not have much in the way with load. This has a cost, so bunch up several like loads on one breaker. Except if the load is critical, like a medium, high, or EHV breaker.

To bacon's point, each of those devices with glowing red lights adds to the battery burden, and needs to reviewed that you have not just crossed the line of capacity/required backup time.
How many times I have heard ' I think we are ok, because it is such a small load we are adding'.

RE: 125V DC Power


Great feedback! Ideally, we would like to be able to put the meter in it's own circuit but unfortunately there are no spare on the 125V DC panel. The units all have their own separate power banks, separate 125V DC panel and the capacity is sufficient after doing some calculations on the loads. The 3 meter that we will be installing for each generating station unit draws in 5 mA of current each, all three would only be 1.875 watts. It has been decided that we will tie in to CKT. 135 for the Tesla 3000 recorder since it performs a similar function to the meter where it just records data and that it is relatively close to where the meter is going to be. We will also be putting in fuses for easy isolation and to provide short circuit protection. I believe the smallest standard rated fuse is 1 amp but I will look into the protection side of it by reading the NEC. I do have experience with doing systems analysis, coordination, short circuit, and arc flash so it should be relatively easy from this point forward. I will also look into the inrush current to ensure that it doesn't trip the fuse when starting the meter.

Thank you for helping out this young engineer. I appreciate all your efforts in making and creating a better well informed community.

RE: 125V DC Power

Having the right size replacement fuse on hand for future replacement can be much more challenging than sizing it right the first time. Be sure that there is a plan in place to stock the appropriate fuses for plant maintenance staff.

The TESLA_3000_manual shows a 5 amp fuse/breaker.

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