Author Topic: DC power system and grounding.  (Read 1686 times)
Roi_hartmann
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DC power system and grounding. « on: May 24, 2017, 01:40:08 PM » Author: Roi_hartmann
Hello everyone.

Some time ago I was able to get Ascom AMS48/1900-4 dc power system. I'm planning to use it to feed my dc powered IT equipment at home but I have some question in mind that I have to figure out first. I've been working with dc powersystems in my job but not that much and it's completely different environment and most of the procedures are describe in some manual.

First one is for the ac feed. Originally it has had 3 phase 3x16A feed but since I dont have such available and only going to use two modules (to get n+1) I planning to run it with two ac feeds. Rectifier modules has similar input connector as regular pc so single phase feed is easy to connect. My home office has a kind of weird electricit implementation since power outlets are are feeded with two different circuitry (which are in different phases of 3-phase feed of my apartment). Since my house was build in 1970's there is no separated ground wire and neutral wire is separated to ground prong in every power outlet. The question is can it do any problem feeding the system with two different feeds since it will connect the ground (and there for the neutral) together in the dc system?

The other question is for the grounding in general. Now since I live in apartment build over 40 years ago there really is not much grounding available. In my work it's so different since there is grounding everywhere and lots of it. My apartments distribution box is old and has only 4x6mm2 feed (3 phase + PEN) I probably should connect a separated 6mm2 ground wire from there to ground the dc system. I'm just still wondering if it's good enought grounding.

At this point there is not gonna be battery backup even thought the dc system has an option for that. The controller can do a smart charging and stand-by charging with temp compensation etc so at somepoint I would want to add a battery string of 30 to 60Ah. I was thinking using vrla or sealed lead acid batteries. I have had an impression that those dont need ventilated room since gas is released only when something is wrong. Is that true and in generally, how good idea it is to licate those inside the apartment (if you dont consider the esthetical side)
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Ash
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Re: DC power system and grounding. « Reply #1 on: May 24, 2017, 04:05:49 PM » Author: Ash
What isolation level is the DC part of your system ?

SELV - Very reliable separation between the DC and the 230V AC. The DC is not connected to Earth/chassis

PELV - Fairly reliable separation between the DC and the 230V AC, The DC IS connected to Earth/chassis

FELV - The DC is in direct connection with the 230V AC (or the rectified line voltage etc)



Since my house was build in 1970's there is no separated ground wire and neutral wire is separated to ground prong in every power outlet

This is TN-C to the receptacle, and is legendarily dangerous

1. If upstream Neutral breaks, the connected device enclosure will become live from the Phase through the device's own impedance. If other equipment is connected to the device (by e.g. data cables), that will become live too

2. If other equipment is connected to the device by data cables, and the other equipment's chassis is connected to intact Earth (or Neutral) on the far end, then in the event of Neutral break in the receptacle for this device, the working current will go through the data cable shields and the other device's chassis. Everything will appear to work normally, maybe except some interference. The data cable is not meant to handle such currents and can heat to the point of damage to devices or catching fire

3. If the devices are on different circuits, the current going from Neutral A to Neutral B through the cable may be dangerous even when nothing is broken, simply because of redistributing the Neutral current between the paths

4. In case 2, if you disconnect a data cable, there will be 230V between the cable and the chassis of the device you pulled it from. When crawling under desks and disconnecting cables it is very easy to touch both

5. In case 3, without anything broken, the current from Neutral to Neutral will cause tripping of the RCD if atleast one of the circuits is on RCD. So for IT use those circuits cannot even be protected by RCD

By all means, repair this. The minimum you can do (which may or may not be allowed, but is safer than what you have now) is disconnect the Earth terminals in the receptacles and leave them disconnected. Then protect the circuits with 30mA or lower RCDs



The question is can it do any problem feeding the system with two different feeds since it will connect the ground (and there for the neutral) together in the dc system?

Yep. It will let the Neutral current from the 2 supply sources redistribute, through the branch circuits on which the receptacles are, equipment power cables and equipment chassis

The more far away the circuits are separate (separate circuits from same panel, from 2 different panels, from 2 different supplies to the building, ..) there are more loads present on each side, and the current going through the connection between the Neutrals may be higher. It may be current from all the other loads on the parts in which the circuits are separated, so by far exceed what the conductors (in the plugged in power cables) can handle. This is a fire hazard

This applies not only to the DC system with RPS, but to anything like PC and monitor plugged into the different circuits as well



The other question is for the grounding in general. Now since I live in apartment build over 40 years ago there really is not much grounding available. In my work it's so different since there is grounding everywhere and lots of it. My apartments distribution box is old and has only 4x6mm2 feed (3 phase + PEN) I probably should connect a separated 6mm2 ground wire from there to ground the dc system. I'm just still wondering if it's good enought grounding

If the PEN breaks upstream from the place at which you split off the PE, your PE will become live. PE can be split from PEN only when you can trust your life to the reliability of the PEN integrity upstream from the connection point

There are standards which describe when PE can be split, with exact specifications. They all come down to reliability and voltage drop of the upstream PEN. They may specify minimum conductor cross section, maximum allowed cable length to an upstream True Earth connection, maximum allowed number of splices, how to connect the split itself and so on. But even when your system satisfies the formal standard requirements, use reason : If you have the slightest doubt in the reliability of your PEN ("it is going through some upstream boxes which i dont have access to", "i dont know where the connection to True Earth is", .. are all very valid reasons), dont take PE from it

If you can upgrade your system with proper Earthing by using local Earthing electrodes, connecting to structural Steel elements of the building's foundations and so on, then make yourself a proper T-T system

(It may well be possible to install a proper and code compliant T-T in your situation. Check what your code book says. But even hacked T-T with unverified resistance to Earth, is far safer than any other hack you could possibly make. Any half decent T-T will let through enough current to trip an RCD, therefore together with RCDs on all circuits provides very reasonable safety)

(Most new installations would be using TN-CS, but when you connect outside PEN to your personal Earth connection, you accept that if PEN breaks far upstream, then the N current from all the block will go through your Earth connection. You might see your 6mm^2 wire glowing and catching fire. In modern systems the practice is to connect local Earth in every building, so there are multiple redundant Earth connections)

If the DC system is isolated and the isolation is SELV grade, it can be left floating. If it is PELV grade it must be Earthed according to standards, but it is still safer to leave it floating than to connect it to anything derived from Neutral or unverified reliability PEN



At this point there is not gonna be battery backup even thought the dc system has an option for that. The controller can do a smart charging and stand-by charging with temp compensation etc so at somepoint I would want to add a battery string of 30 to 60Ah. I was thinking using vrla or sealed lead acid batteries. I have had an impression that those dont need ventilated room since gas is released only when something is wrong. Is that true and in generally, how good idea it is to licate those inside the apartment (if you dont consider the esthetical side)

I guess the batteries do outgas a little, even "sealed" ones

The main thing they outgas is Hydrogen and Oxygen from the Water content in the electrolyte. Both are harmless to health

The secondary thing - Is it possible that fumes from the Sulfuric acid come out too ? Those can be a greater concern, though i doubt there would be any significant quantity from 1.5..3kWh worth of batteries. If there is something, i think it might be possible to make a cabinet for the batteries with solid powdered "getter" inside to catch most of it. something like a tray with Sodium Bicarbonate would probably be fine as a getter

Another possible concerns :

 - Amount of Energy stored in such battery array - If that shorts out it can put out quite a fire

 - Amount of smoke such array can produce if it shorts out. In an enclosed room in which you are sleeping, there may be risk of asphyxation. I dont know how fast would an optical smoke detector catch the smoke from batteries to wake you up
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Re: DC power system and grounding. « Reply #2 on: May 26, 2017, 01:30:02 PM » Author: Roi_hartmann
Just as I thought. I have to think how can I make this even somewhat safe.

I read the manual of the system and did not find any mentioning about the isolation of dc part. This system is supposed to be grounded and it is assumed the dc is also connected to the ground. And since this is telco dc system the voltage is -48v. So it's positive ground for dc.

I know TN-C sucks. I've had some past problems with ground loops in audio equipments. There are no RCD's installed. since the distribution board is so old there are no possibility to instal one in any easy way. You also can't use the dc system behind RCD since there is high leakage current in it(I guess that's common thing with theese)

The closest earthing connection in electrit system I know is in service building of this apartment complex. There is main distribution board and kwh metering. Thats where the maind feed of my apartment comes with dedicated cable. So at least there is grounding point.

Local earthing electrode would be nice but I dont think I be able to get permission from housing cooperative for that.

Regarding to possible battery backup, I was thinking 4 pce of 30-60Ah (exact size not decided) batteries connected in series to get one 48v string. I know burned batteries are pretty nasty since I have cleaned such mess couple of time but it was much bigger array of almost 400Ah. I don't have much load so I can manage with smaller capacity.


Here are some pictures. I know the system I overkill but I got it free and it's passive cooled so it does not make anynoise unlike most of the newer systems. Each module is able to feed 1900w. I did some modifications like installed low voltage disconnection contactor for the battery connection and changed smaller battery circuit breakers. I have to get some schneider circuit breakers to replace the missing ones. I removed the plastic panels so you could see inside.

http://www.whitenightcape.com/rectifier/front.jpg

http://www.whitenightcape.com/rectifier/open.jpg

http://www.whitenightcape.com/rectifier/inside.jpg

http://www.whitenightcape.com/rectifier/panel.jpg
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Re: DC power system and grounding. « Reply #3 on: May 26, 2017, 06:18:05 PM » Author: Ash
Remove the TN-C jumpers in the receptacles

Protect everything with RCDs

Install Earthing :

 - Without it the RCD will only trip on a fault when you touch the equipment and get >30mA of thunderbolt attack. I.e. it will save you, but you won't like the experience. With Earth connected, the RCD will trip as soon as the fault appears on Class I (metal chassis connected to PE) and PELV (extra low voltage connected to PE) equipment

 - Without it most equipment will be at ~110V potential to Earth, because of the Y caps in EMI filters. The current the Y capacitors let through is sufficient to damage equipment when plugging/unplugging a cable (if some data pin makes contact before the Earth pin or cable shield), and sometimes to give you a modest Pikachu

The TN-C provides the same functionality as Earth, but with normal equipment which have fairly reliable isolation even for Class I, the TN-C's dangers exceed the dangers of having no Earth



DC is connected to PE - So it is a PELV system :

 - SELV Safety Extra Low Voltage - Extra Low Voltage system with safety provided exclusively by isolation between the 230V and the ELV, The isolation is made very reliable and (supposedly) cannot fail. If it fails, there is no other protection

 - PELV Protected Extra Low Voltage - Extra Low Voltage system with safety provided by connecting the ELV to PE. There is full isolation between the 230V and the ELV and no leakage there (except the Y capacitors), but the demands for isolation reliability are normal (not very high as in SELV). The PE provides protection in case the isolation fails

 - FELV Functional Extra Low Voltage - Extra Low Voltage without isolation, for example if it is derived from 230V with a voltage divider. FELV cannot be connected to PE because it will mean a connection between 230V circuit and PE



With very rare exceptoions, No modern equipment is designed to have significant leakages between 230V to PE. There is isolation between the 230V circuit (both Phase and Neutral) and the PE. Your RPS is not supposed to have any leakage that would trip the RCD

Equipment with leakage high enough (by design, not because of a fault !) to trip RCDs, must be connected to very reliable PE in order to prevent danger of electric shock or fire. The only example that comes to mind is electrode boilers. I'd expect that such stuff is installed with bolted connection to PE busbar or potential equalization system, definitely not suffice with the PE of a branch circuit at home

In short - Nothing you use at home is supposed to trip the RCD



Whats in your panel ? Is everything else there okay and in good condition ? Can you post a picture ?



That main distribution board does not necessarily have Earth. It may be connected to the PEN, but this still does not say anything about the reliability of the PEN up to it (and about the safety of touching the distrubution board's enclosure, if lights start to dim or brighten up and you rush to the board to switch off the main...)

If you can verify where is the actual Earth connection (to True Earth), the reliability of the PEN from there, and that it meets other requirements, then it might be adequate to take PE from. Otherwise dont use it as PE



If any structural Steel of the building foundations is accessible, this is a good candidate for use as the basis for an Earthing system

As long as the Earth connection can sink >30mA, it will work well enough to trip the RCD on a fault. Much less is sufficient to eliminate the voltage on equipment from Y capacitors. So, any fair enough Earth connection (used in T-T) together with RCDs provides reasonable safety

But just Earth connection with unverified parameters is not compliant to any standards. Standards require measurement of the resistance to the main conductive mass of the Earth, and interconnection with other Earth providing networks like the Water pipes entering the building



The system is nice



Carefull with disconnects on the DC !!!

As DC does not have zero crossings, it is quite hard to extinguish DC arcs. Contactor that is rated for X amps AC, may not be able to extinguish the arc when opening a DC circuit, even if the current and voltage are well below the AC rating. If this happens, it will burn up with flame in seconds

Check the DC rating for every switching device you use with DC



Are there any connections with a comb bar ?

If no, you can put in any breaker of your choice, does not have to be Schneider. (Unless you are looking for breakers with the matching appearance, but present day Schneider Acti9 look different from the Merlin Gerin)

If yes, and you cant find Schneider, look for old Chint :



There were largely 2 generations of the Multi 9 :

The old C32*..C45* (its not the current rating, its the version. It is written in the line below the Orange strip on the breaker)


The later C60* :


You got the later and better version. Main difference is that the C60* have instantaneous make - When you switch the breaker ON, the contacts make connection quickly, minimizing arcing. (you can hear a click when switching the breaker ON slowly). The C32*/C45* and most breakers from other manufacturers dont have this feature

Most present day Chinese breakers are clones of the C32*

Few Chinese breakers are clones of the C60*. The ones i know are Chint old series NB1-63 (before 2006, when Schneider sued them for patent infringement and they moved to cloning the C32* like everyone else)

Same as the Multi9, the Chint with thin handle/4 rivets is the C32* and with wide handle/6 rivets is the C60* (though i seen other Chinese manufacturers make C32* with wide handle)

Chint have fairly decent quality, and the different Chints fit well with the C32* and C60* Multi 9's respectively on the same comb bars. (Though i'd stay away from all the other Chinese breakers)
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Re: DC power system and grounding. « Reply #4 on: May 29, 2017, 02:51:35 PM » Author: Roi_hartmann
I have to put this project on hold for a while. I'm having a little operation tomorrow and have to spend few weeks on hospital but I will check back when the time is right.
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