from the Forum department...
Measurement of Battery Bank Voltage for Mark-IV
 Posted by Peter Wale on 16 July, 2009 - 3:45 am
Pls help me to clear my doubts. we are using 55 nos. of cell for battery charger system of Mark-IV panel.

At Battery bank (BB), when we measure the voltage, it found as follows

between final output (02 terminals) = 130 vdc
between -ve terminal of BB & GND = 65 vdc
between +ve terminal of BB & GND = -65 vdc

How? that i don't know. Pls help me to understand. This supply is going to mark-IV panel, where it is once again divided by resistor network.

 Posted by CSA on 16 July, 2009 - 4:35 pm
This is not peculiar to the Mark IV; all Speedtronic panels since the Mark IV have the same input circuit, for all intents and purposes, though that seems to be changing somewhat with the Mark VIe.
`    -------o  o-------------------    |     BP  SP  |              |  -----           R   |        ------   ---            |---|| GND  |      |  -----           |   |       | MkIV |   ---            R            ------    |     BN  SN  |              |    -------o  o-------------------`

BP = Battery-Positive
BN = Battery-Negative

MP = Speedtronic-Positive
MP = Speedtronic-Negative

R = Voltage Divider Resistor

First, consider the "connections" between BP-SP and BM-SN to be open. If you use your voltmeter to measure between BP and BN, you should see approximately 130 VDC.

If you use your voltmeter and measure the DC voltages at BP and BN with respect to ground, you should see some "strange" readings. Strange, because, if the battery (and it's charger) are ungrounded, which they should be, then when you apply the meter leads the voltage will probably appear to go very high and then quickly decay to something near zero volts, but not zero volts. If the reading goes to 0.00 VDC, that would indicate that the lead is grounded, and Speedtronic panels all want an ungrounded supply. If you measure the other battery terminal, you should see the same thing: a high reading, quickly decaying to near zero, but not zero.

If you see an even split of voltage, and the battery charger is powering the battery, and the connections between BP-SP and BN-SN are open, that indicates the battery charger ground detection circuit is "active", and that can interact with the Speedtronic ground detection circuit and cause all manner of nuisance ground indications. If one of the voltages goes to zero under the conditions cited, then that indicates that terminal is grounded, and the battery supply to a Speedtronic is to be ungrounded.

Let's consider for a moment that all other devices that are connected to the battery are disconnected from the battery. In other words, all the breakers in the 125 VDC distribution panel are open, *and* all 125 VDC motor starter circuit breakers are open. Some plants use a common 125 VDC battery for the Speedtronic and for other purposes as well; let's consider that all circuits and devices powered by the 125 VDC battery which also supplies the Speedtronic are disconnected from the battery.

Now, when the connections between BP-SP and BN-SN are made up, then the ground reference circuit of the Mark IV is placed across the battery terminals. If there are no grounds on the devices connected to the Mark IV then the voltages will be evenly split with respect to ground, just as you have described. If there is a ground on any of the devices powered by 125 VDC connected to the Speedtronic (contact inputs; solenoid outputs), then that will affect the voltage split indicated by the voltmeter readings.

When the Speedtronic ground reference circuit is in place across the battery terminals, it is sensing *ALLgrounds on all circuits and devices connected to the battery! So, when the Speedtronic annunciates a 125 VDC Battery Ground alarm, it does NOT mean that the ground is on a device connected to the Speedtronic. The ground can exist on any circuit or device connected to the same battery that is supplying the Speedtronic. So, when all the circuits and devices are connected to the battery (and charger) and if the Speedtronic indicates a 125 VDC Battery ground, it can be on any circuit or device connected to the battery; not just on devices or circuits connected to the Speedtronic. This was an inexpensive method of monitoring the battery for grounds, using the Speedtronic and has caused more misunderstanding than could be imagined.

If you are saying that the battery is disconnected from the Mark IV and you are still reading an even voltage split with respect to ground, then that likely indicates that the battery charger ground detection circuit is connected and operational, and that will usually cause problems when the battery is eventually connected to the Mark IV. The two voltage divider networks will be in parallel, and the Mark IV ground detection circuit doesn't like that.

If you are reading the even split with the battery connected to the Mark IV, it still might mean the battery charger ground detection circuit is connected and operational; the only way to know for sure is to isolate the Mark IV and check the battery terminals.

 Posted by Peter Wale on 18 July, 2009 - 2:12 am
Dear Sir,

Sorry for the delay due to ongoing shutdown activities & Sorry following errors in my query

between -ve terminal of BB & GND = -65vdc
between +ve terminal of BB & GND = 65 vdc

We have done the battery capacity test due to availability of Mark-IV shut down activities. The Charger was isolated & only battery terminal is available i.e. BP & BN only with respect to earth.

As my readings are perfect as per requirement, my conclusion is battery is ungrounded. Then we connected battery to chargers, took the readings the same found O.k. with almost no change. After that we connected Mark-IV panel breaker for charging. The terminal voltages are found O.K. In addition to that we have connected DCEOP with Battery Charger.

Also I want to know that is there any specific reason to use the ungrounded supply for the Mark-IV.

Besides in situation like earth fault, one terminal is already earthed, say, solid +Ve earth at fire detector. Now another terminal -Ve terminal is earth at another fire detector or at any device. ---- As per the theory, both battery bank terminal BP & BN are shorted,what would happen to Mark-IV panel supply.

 Posted by CSA on 19 July, 2009 - 12:59 pm
Your readings are *NOT* as per requirement. If the battery was disconnected from the charger and from all loads, and you measured +65 VDC from BP to ground and -65VDC from -ve to ground, and these voltages were stable (not changing) then there is a ground reference circuit somewhere across the battery terminals similar to the one used in the Mark IV <PDM>.

This does not mean there is a ground; just that there is a ground reference circuit somewhere across the battery terminals. If there were no ground reference circuit and the battery is ungrounded (which it sounds like it is), then when you try to measure a voltage from either battery terminal to ground there won't be any stable voltage with respect to ground; the voltage will be "floating" and not perfectly split at you describe.

The voltage can only be perfectly split as you describe if there are no grounds *AND* there is a ground reference circuit.

If there is a ground on one leg of the battery and then a ground develops on the other leg of the battery that's just like a short across the battery terminals! There will be sparks and cell caps popping off and likely fuses will be blowing and the Mark IV will likely lose 125 VDC power.

 Posted by Peter Wale on 22 July, 2009 - 8:01 am
Dear Sir,

We are mesuring voltage at BP & NP with respect to earth terminal. The reading is evenly half of the voltage between BP & NP. And battery bank is totaly isolated from Mark-IV & even Charger also.

 Posted by CSA on 22 July, 2009 - 7:04 pm
Take a battery, any battery (a 9-volt consumer battery is as good as any) [a battery used for a GE Speedtronic panel isn't anything special], and make sure the case isn't touching ground and neither terminal is touching ground and use a voltmeter to measure one terminal with respect to ground. Record that reading. Switch the voltmeter lead to the other terminal with respect to ground and record that reading.

If neither terminal is grounded, and there is no ground reference circuit, then the "readings" should be pretty "useless", as in not rated voltage, and not 0.0000 VDC (my recently calibrated Fluke 87 went to 0.0004 VDC), and not stable (not repeatable), and not half the rated voltage either. (When I grounded the terminal my meter was connected to, the voltage was immediately to a "solid" 0.0000 VDC. When I checked the other battery terminal to ground, I read a "solid" 8.8954 VDC.)

If you're still not convinced, put two batteries in series and repeat the test above. (By the way, have you ever carefully opened a 9-volt battery? it's pretty interesting!) You'll get the same results.

I knew at least two GE field engineers who fervently believed that an ungrounded battery would read a perfect split with respect to ground. And it will--if, and only if, there is a ground reference circuit with resistors of equal value in series with the center tap grounded.

So, they would install their own resistor network on the battery terminals and then connect the battery to the Mark IV or Mark V. (Of course this caused an interaction with the ground reference circuit on the Speedtronic panel, but they were convinced that because they couldn't get repeatable stable readings of half rated battery voltage that the battery was not configured properly. So, they added their own ground reference circuit to obtain the readings they wanted to see before connecting the battery to the Speedtronic panel.)

Your battery is ungrounded, but I suspect that there is a ground reference circuit somewhere to allow the voltage readings you have reported with respect to ground. I don't know any of any other way for that to happen, unless someone grounded the center of the two "middle" cells of the battery, which would still be incorrect. And likely lead to sparks if there were any grounds in the field.

 Posted by Andrew on 21 September, 2012 - 5:22 am
> This is not peculiar to the Mark IV; all Speedtronic panels since the Mark IV have the same input circuit, for all
> intents and purposes, though that seems to be changing somewhat with the Mark VIe.

> If you are reading the even split with the battery connected to the Mark IV, it still might mean the battery charger
> ground detection circuit is connected and operational; the only way to know for sure is to isolate the Mark IV and check the battery terminals.

I know exactly what your saying... did you find out why a terminal to ground gives you exactly half the voltage?

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