the effects of 125vdc ground faults

Good morning everyone. please what could be the dangers of 125vdc ground faults to the cards on Markv speedtronic controller and field instruments. i know this topic has been on for a long time now, but i have not seen where the dangers to instruments and mark* controller is mentioned. thank you
If you've read some of the many previous threads on this topic you know that the Mark* is designed to operate with a ground on either leg of the 125 VDC supply--the positive leg or the negative leg. In fact there can be multiple grounds on one leg--and the Mark* will operate normally. In most circumstances. (Sometimes a ground which develops on one leg can seem to move to another leg--particulary when the ground is on a discrete (contact) input circuit. In some cases this can cause the Mark* to act very strangely and intermittently as a contact opens/closes. It's rare, but it does happen on some older Mark* turbine control panels, including some early Mark* Vs.)

The real danger to the Mark* and it's components (printed circuit cards and the like) occurs when a ground (or grounds) on one leg is left unattended or unresolved and a ground occurs on the other leg. Then a whole lot of arcing and sparking happens and things get burnt and destroyed. When there are grounds on both legs of the 125 VDC source it's like a "dead" short across the source (that is, like touching a metal wrench to both legs of the 125 VDC source. Even a 14 AWG wire can carry more than the 15 amps it is rated for and many fuses in a Mark* V are rated for 20A such as for solenoid circuits.

Yes; there are fuses--but many of them (especially in the Mark* V) protect multiple circuits so they are larger than they would be if a single fuse protected a single circuit.

The alarm 125 VDC BATTERY GROUND when annunciated by the Mark* refers to a ground on ANY CIRCUIT/LOAD POWERED BY THE 125 VDC BATTERY (source--it might be a DACA, a device that converts AC to 125 VDC) THAT SUPPLIES POWER TO THE Mark*. A ground detection circuit CANNOT distinguish between grounds on devices/wires connected to the Mark* and devices/wires which are NOT connected to the Mark* but powered by the same 125 VDC source. I have been to many sites with grounds which existed for a LONG PERIOD OF TIME and were not resolved and had multiple grounds on the same leg of the 125 VDC and were ultimately found to be on devices NOT connected to the Mark* (fire detection/discharge systems; plant security system; step-up transformer protection; black start power systems; etc.). Simply because the Mark* is annunciating the 125 VDC battery ground alarm DOES NOT MEAN the ground exists on a device/wire connected to the Mark*. This is a huge assumption many people (including Operations Managers and Plant Managers make and force on their technicians: "It MUST be something connected to the Mark* because the Mark* is annunciating the alarm!").

Devices connected to the Mark* which have 125 VDC applied to them from the Mark* include discrete (contact) inputs (limit switches; level switches; pressure switches; temperature switches; vibration switches (on skid-mounted cooling water fans, for example, where a LOT of 125 VDC battery grounds occur), etc.), 125 VDC solenoids (fuel trip solenoids; pneumatic solenoids; etc.), in some cases fire detection and fire extinguishing agent discharge systems (fire sensors; discharge solenoids; alarms/strobes; etc.).

NOTE that devices like servo-valves and pressure transmitters and LVDTs and speed pick-ups and thermocouples and RTDs and vibrations sensors CANNOT cause 125 VDC battery grounds. Yes; many of those voltages and signals are derived from the 125 VDC source powering the Mark*--but they are isolated from the battery and even if they develop a ground they WILL NOT cause a 125 VDC battery ground alarm to be annunciated by the Mark*. The Mark* doesn't have the capability to sense grounds on all those circuits, and is some cases is designed to operate with grounded sensors (RTDs and T/Cs come immediately to mind) so even if a ground is found on one of those devices/circuits it will not be causing a 125 VDC battery ground alarm. (I have seen people using a multimeter to check EVERY screw terminal in the Mark* panel to ground--and find grounds on RTDs and T/Cs and spend days troubleshooting them and the 125 VDC battery ground never goes away, and they are extremely frustrated and angry when told those devices/circuits can't cause 125 VDC battery ground alarms!)

So, again: A single ground--or even multiple grounds--on one leg of the 125 VDC source (which is also used to provide power to many circuits and devices--and not only on devices/wires/circuits connected to the Mark*!) is not a problem--the Mark* is designed to operate fine so that the ground can be found and resolved. The real danger comes when there is a ground or grounds on one leg of the 125 VDC source and another ground develops or occurs on the other leg of the 125 VDC source. That's when the arcing and sparking and smoking in some cases occurs--and the real problems begin. Often a ground occurs because the insulation on a wire or cable is damaged--usually because of heat but sometimes because of vibration/rubbing/chafing. A ground will occur on one leg and if the insulation of the other leg occurs because of the heat or vibration/rubbing/chafing, well, then, it won't be very pretty.

Hope this helps!

Imagine if one leg of the 125 VDC source to the Mark* was grounded--like the 12 VDC battery in a car. If a ground or multiple grounds occurred on the negative leg--it is impossible to know when or where. BUT, if a ground occurs on the positive leg--well, again, that's like throwing a piece of metal across two bare conductors of the DC source legs. There's going to be arcs and sparks and probably some smoke. Most circuits in an automobile have fuses to protect individual circuits and devices and prevent bigger problems--but most Mark* turbine control panels do not, and many turbines and associated auxiliary devices have MANY MORE sensors and wires than a typical automobile (though I imagine some EVs (electric vehicles) can have as many or more sensors and wires!).