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Speed Pickups Sensing Speed at L14HR
When machine is at rest, speed pickups are sensing speed up to 3200 rpm
By chiranjeevi on 28 April, 2017 - 9:16 am

We have MARK VIe system for frame 5 machine. GT has tripped due to command activation form CO2 sys.

after that speed pickup, some times one pickup sensing up to 3200 and
some times two pick ups are sensing up to 3000 rpm. due to this speed diff trip, composite in PPRO is activating.

was there any problem with sensor cable shield. Moderator's Note: I think this is supposed to be a question.

we are removing the pickup and keeping a side, but during cranking we
are connecting back in mark vie panel. then all pickups are showing same.

can anybody explains?


You are most likely correct--there is some issue with the wiring (should be TSP (Twisted, Shielded Pair) wiring, with a drain wire to shunt electrical noise to ground (earth)), most likely with the shield drain wires. The convention is the shield of any circuit of twisted, shielded pair should only be terminated at ONE end. So, if the wiring between the speed pick-ups and the Mark VIe is one length of TSP, the drain wire should be terminated to ground (earth) at one end only.

If the circuit consists of multiple lengths of TSP, each terminated in a junction box, the drain wire of each length can EITHER be connected to be continuous for the entire circuit and terminated at one end only, OR one end of each length of TSP can be terminated to ground/earth in each junction box.

Once the shaft is spinning and the speed pick-ups are generating sufficient voltage it sounds as if that's enough to overcome the electrical noise which the shield drain wire(s) are not shunting to ground (earth).

The other problem which may be occurring is that the cables between the unit and auxiliaries and the Mark VIe may not have been properly segregated during construction, and these low voltage level cables are run in conduit or trays or vaults over some distance with wires/cables which carry higher voltages/currents.

You have never told us if the Mark VIes were provided new with the turbines when they were new, or if an older control system on an older turbine was upgraded (retrofitted) to a Mark VIe.

Based on the information provided, that's about all that can be said.

I gotta tell you, though, that what you are doing can also cause the Mark VIe to trip the unit, because the Mark VIe is also monitoring the rate of change of speed signals and if the rate of change exceeds some default value a turbine trip will be initiated (even if the unit isn't firing, but just cranking).


Here's an example of how to use the 'Search' feature of So, we were talking about drain wire grounding (earthing) (weren't we?), and maybe it could be a little clearer with a drawing. I typed the search term:

     +"drain wire"
into the 'Search' field, clicked on Search, and this was one of the results:

A LOT of information exists here on, and many less esoteric questions (that you have been asking most recently) have been answered--some many times in the dozen or so years the GE heavy duty gas turbine control community has been active here on And, all that information is accessible from the 'Search' function. (It's suggested you look carefully at the search term above, and use the Search 'Help' to learn the syntax and format of effective searching on

Happy Searching!

By oskachiller on 28 April, 2017 - 11:57 pm
1 out of 1 members thought this post was helpful...

First : You should be confirm distance of this sensors its = 3.2 mm or not? if you setup this sensors closeup with object too much its will be reading same this value.

Note: this problem occur after you have activity with engine or not? (Shutdown period or engine change or overhaul engine.)

By chiranjeevi on 30 April, 2017 - 2:51 am

distance b/w sensors was already fixed by the shaft casing. it is more than 10cm. are u talking about sensor cable distance?