MARK V LCCB Monitor Blackout


Thread Starter


Dear all,

our LCCB monitor for mark V core <R>, <S>, <T> blackout. Upon checking fuse at each TCPS was found blown. Replaced fuse, ON back power supply fuse blown again.

Please advice were to start...

What Process Alarms were active prior to this problem?

What Diagnostic Alarms were active prior to this problem? If you will provide the Process- and Diagnostic Alarms we can probably provide more help.

When did this problem start? After the panel was powered down during a maintenance outage? After a plant black-out caused, by, say, a lightning strike? Or a serious grid instability issue? Did the turbine trip immediately prior to finding the LCC displays all black?

The TCPS cards are powered by 125 VDC and the primary source of the 125 VDC should be from a battery and battery charger, and the various voltages produced by the TCPS cards are all referenced to the same earth (ground).

Does the unit also have an AC-to-DC converter, usually called a <DACA> which is powered and providing "back-up" power to the Mark V? Is the <DACA> powered by a UPS?

Have you measured the 125 VDC inputs to the Mark V with a DC voltmeter with respect to earth (ground)? If so, what is the positive DC voltage with respect to earth (ground)? What is the negative DC voltage with respect to earth (ground)?

While Mark V Speedtronic turbine control panels can operate with one or more grounds on the SAME leg of the 125 VDC supply (positive OR negative), it's not recommended to run the system for long periods of time with "a" ground (or grounds) on one leg of the other. Most of the time when I have seen this problem it's been because there is, and has been, a serious 125 VDC ground problem on the Mark V for a long time (sometimes, years--yes, years).

There's this myth that troubleshooting grounds on Mark Vs is impossible--and that's what it is, a myth; a falsehood. Troubleshooting grounds is not always easy, but neither is it impossible. Neglecting grounds is never a good idea, and just because the system will run with one or more grounds on one leg doesn't mean it's intended to be run or should be run that way. It's just asking for troubles, like the one you are experiencing.

The other most common cause of this kind of problem is that somehow AC has been "connected" to one leg of the DC supply, and when there is also a 125 VDC battery ground present this leads to all manner of "unusual" and "nuisance" problems. All of which are blamed on the Mark V, but all of which are due to neglect--neglecting to find and resolve the cause(s) of the grounds.

You can take a digital voltmeter and put it in AC mode, connect one lead of the meter to ground and measure the DC input to the Mark V at the terminal board of the <PD> core; I believe Terminals 1 and -2 are the positive side of the 125 VDC supply, and Terminals 3 & -4 are the negative side of the 125 VDC supply. Measure each leg to earth (ground) with the voltmeter in AC mode--you should see very little, if any, AC voltage. If you see more than approximately 40 VAC with respect to earth (ground) on either leg, then you need to determine where the AC voltage is coming from on the leg with the highest AC voltage with respect to earth (ground).

Sometimes, the output capacitors of battery chargers also fail, and cause a high ripple on the DC output which can also cause problems with TCPS cards. You will need an oscilloscope to check for this.

Never--under any circumstance--disconnect the battery itself from the output of the battery charger and then power the Mark V with only the battery charger. That can be very destructive to Mark V power supplies, if not immediately, then it can weaken components on the TCPS cards that will eventually fail.

If there is one or more grounds on either leg of the DC supply, the best place to start looking for them is in junction boxes and devices which are exposed to ambient weather conditions (that is, junction boxes which are not inside any compartment or enclosure). Poor construction practices were used on many sites that allowed penetrations into junction boxes from the top of the box, and if not properly sealed (and most weren't) they water can get into the boxes through the top penetrations. Also, many times junction boxes and switch enclosures are not properly closed after entry, or become rusted and leaky. These are the most likely source of grounds. And, most grounds are usually found in external junction boxes and switches.

Please write back to let us know how you fare in resolving the problem.
Dear Sir,

If you LCCB and TCPS is faulty and you want them repaired let me know. We have a repair facility.

Also, do you want TCPS or LCCB card? I can supply from stock so that you don't face any downtime in the plant.

Do drop your e-mail id and i will send you my profile.

Dear All,

sorry for the late reply, this incident happen while the unit was online and what puzzling us no process or diagnostic alarm triggered. Unit was running fine. We have a shutdown a day after that and the culprit is shorting from one of the proximitor probe at TBQD <S> core


Thanks for the feedback.

I found an old version of GEH-6195, the Mark V Application Manual, and in Appendix D, Fig. 28, it shows the N24 VDC power supply for proximitors from each control processor (<R>, <S> and <T>) is high-selected using diodes on the TBQD card. So, it's conceivable if the the short was severe enough it could affect the N24V power supplies of all three processors by blowing the fuses on the TCPS cards.

Are you able to tell us how the proximitor came to have such a serious short? Was it the proximitor (the device that connects to the Mark V and to which the proximity probe is connected with the special cable/connector) or the proximitor probe (the device that measures the distance from the shaft and connects to the proximitor with the special cable/connector)? Was it caused by excessive temperature (heat) on either the proximitor or the wiring connecting the proximitor to the Mark V or the proximitor to the proximitor probe? Or was the wire/cable somehow mechanically damaged (cut; pinched)?

I believe the DENET (Data Exchange NETwork) also uses the N24 VDC power supply--but that's just a swag (Scientific Wild-Arsed Guess). That could affect the ability to transmit alarms (Process and Diagnostic), though it would seem that the unit would have tripped because there was no exchange of information between the processors because the DENET wasn't working.

Anyway, thanks again for the feedback--and any additional information about the failure would be very much appreciated and helpful to many people who read these threads, both now and in the future.