DC power for communications site

Oops.

I should have stated that the DC load will be about 75 amps at -48VDC.

Most DC breakers are "trip free" design. Meaning they will trip even if the toggle is physically held in the ON position.

Resetting requires you to first move the toggle from the tripped position to fully OFF, then move it to full ON position.

This is not easily done with a simple solenoid plunger. You need a more sophisticated motor operated device capable of bi-directional movement.

"Back Feed" is an ugly concept. You never know where it has been. Think of it as the battery powers the equipment, and the rectifier charges and maintains the battery. When the AC power fails the equipment still operates on battery power, and when the AC power is restored the rectifier recharges the battery. Sometimes the battery is so low that the rectifier must power the equipment and recharge the battery. Setting the current limit on the rectifier is very important.

Don't think of it as backfeed. That implies power is flowing in an unintended and undesirable direction. Instead, consider that you actually have 2 parallel sources to the panel, the battery set and the rectifier shelf. You are simply providing a main supply breaker in the panel for each source. It's really no different than your local utility having multiple generators operating in parallel, connected to the same bus.

I have 25+ DC power systems at generating stations and electrical substations, and every one of them is connected the way your vendor describes. Here is a typical schematic.

The battery is connected to one breaker of the distribution panel, and the charger (in your case, the rectifier shelf) is connected to another breaker. The power doesn't care which way it flows in the panel.

This design greatly improves the reliability of the DC system, and makes maintenance much easier. If you need to work on the rectifier shelf, open the panel breaker to the rectifiers and let the battery carry the load until the rectifiers are restored to service. If you need to replace the battery set, open its breaker and let the rectifier shelf supply the loads until your maintenance is complete. Downtime for your critical loads is virtually eliminated.

Thanks for the re-assuring comments. We are looking at Valere power shelves and others. Do you recommend any DC power shelf vendors?

Now on to item two, remotely resettable DC breakers. I have only seen them in milliamp applications. Do 25- 50 amp units exist?

Thanks again,

jh

PS - Great tag line re: Experience.

Anything can be operated remotely if you really want it. Even the small molded case breakers you would use can be solenoid operated in most cases. However, you should remember that the breaker only trips to protect against an overload. Unless you have some really good remote monitoring, I'd recommend that you inspect the equipment to identify the cause prior to resetting the breaker. Otherwise, you may burn something up, causing damage to the communication equipment that's the whole reason for the installation in the first place.

Yes we have excellent analog and digital monitoring, including voltage and current readings. We may want to retry a circuit briefly and note the readings. Since there are radios involved with their inherent lightning magnets we could get a pulse that could affect us even with the ground rings.

Please tell me more about the solenoid reset. Is it off the shelf or a homebrew device?

Thanks,

jh

Viewed the Square D page and it looks like it is only spec'ed for AC circuits.

That was also my impression of the Square D website. None of that equipment is suitable for DC operation. "Backfeed" and "rectifier" are two words that conjure up visions of antiquated old technology approaches. I hope it's a case of legacy terminology not actual reflection of what kind of equipment is used. Rectifiers implies rippled and poorly regulated DC. UGH!

Back-up battery banks are suceptible to damage from inappropriate charging equipment. By design and application most backup batteries spend months on "float charge".

Unless the charger (rectifier) is very sophisticated, it will damage the batteries. During my career in a power utility job we maintained about 100 such remote sites for the RTU and line switching equipment. We suffered repeated battery failures because the charger manufacture did not understand how to sustain sealed back-up batteries,

Regarding remote operated breakers. The best DC application models I have seen were made by E-T-A . I heard that Airpax has also recently announced such a product for DC applications like the yachts, RV, and off-grid solar power homes I now design systems for.

Many of the OFF-GRID systems are 48V DC so this is where you will likely find the most variety in product choices as of-the-shelf equipment.

Rectifier is a legacy term that manufacturers still use for large DC power supplies. The ones I am looking at regulate within +/- 1%, with noise level under 20mv RMS.

But now that I am shaking out the cob webs it is clear to me that the proposed design was flawed. The batteries need to be connected to a float charger and then connected to the loads via power diodes.

We have now found some remote control PDUs, one source is specpower.com.

jh

Back feed is not the right solution. Normally when there is an outage the battery takes over.We have sensors to ensure that once battery voltage goes down to a preset value it is stopped. Normally supply should restore as batteries can last for quite some time.We have a system of a diesel genset back up with auto mains failure control. To prevent DG from starting for short outages a sensor above for monitoring battery voltage is provided.So at that point if supply is not resumed DG starts.I

Try this site http://www.electroauto.com/catalog/circtbkr.shtml for a look at a compact 250 amp D.C. circuit breaker used in electric auto design.

Although this breaker is manually operated you could, as another contributor suggested, toggle this style with an auto door lock linear motor or a solenoid. Actually, because of it's ability to reverse motion by merely reversing the electrical supply polarity, an auto door (or trunk) lock linear motor is preferable.

Try this site for "pop' door motors and kits, http://www.a1electric.com/ . Hope this helps.