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Why MARK 6 supply is made off during any hot (welding, grinding) job in gas turbine?
By MARKBEGINNNER on 20 April, 2018 - 6:58 pm

Why MARK 6 supply is made off during any hot (welding, grinding) job in gas turbine? Is it related to possibilities of ground faults in Mark 6?

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>Why MARK 6 supply is made off during any hot (welding,
>grinding) job in gas turbine? Is it related to possibilities
>of ground faults in Mark 6?

There are people who will weld or grind on or around a turbine being controlled by a GE Mark* turbine control system that will not always take simple precautions--such as placing the welding ground lead very close to the area where the welding will be done to minimize circulating currents. Grinding has also caused some nuisance issues, but rarely so unless it's done near the control panel and with the doors open and sparks flying into the control panel.

The Mark VI and Mark VIe are both designed to be able to have a functional earth ("instrument earth") and protective earth ("safety earth") system to protect the turbine control system. However, the presence of two earthing (grounding) systems DOES NOT mean that proper earthing (grounding) practices were used in the field when mounting and terminating devices and wires (shields, usually). Also, many functional earth grids are small and in poor soil can be very problematic.

It should not be necessary to power down a Mark* turbine control panel when welding or grinding is to be done on the turbine--as long as proper, simple precautions are taken. The biggest precaution is to place the welding ground lead as close to the area where the welding is being done as possible. If that means some paint has to be removed to do so--then so be it, and the paint can be reapplied if required.

But, again--this is true only if proper earthing practices were followed when the plant was constructed, and also afterwards as new equipment is added. And, while it's not often done the earthing grids should be regularly checked for proper functionality.

But, since it's nearly impossible to tell if proper earthing practices were followed during construction, and afterwards, then it's common for the precaution of powering-down the Mark* panel to be done just as a safety precaution in case simple precautions are not followed by those performing the welding, and, again, because it's often not known well proper earthing practices have been used (both during plant construction) and afterwards and because most earthing systems are rarely, if ever, tested after initial construction.

Hope this helps! In general, if proper welding practices are followed and the grinding isn't a long and drawn-out job, it's not really necessary to power down the Mark*--but many people err on the side of caution, which is always a good thing.

thanks for the response sir.
i have further questions-

1. how does circulating current reduce on placing the welding ground nearer to the place of welding?

2. what will happen if, circulating currents are high, proper earthing methods not followed and mark is still ON during welding job?

>1. how does circulating current reduce on placing the
>welding ground nearer to the place of welding?

If the welder ground lead is not placed close to the place where the welding is to be done the current between the welding rod "stinger" and the welder ground will have to travel the distance between the two. If this is a long distance and there are any control system earths in that path then the "circulating" currents can cause the earth potential to be elevated and unintentional currents to flow through the control system earth leads. This is NOT good for ANY control system.

>2. what will happen if, circulating currents are high,
>proper earthing methods not followed and mark is still ON
>during welding job?

See above.