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Mk 2 Speedtronic Control
Question about Mk2 Master protection permissive "4" circuit.
By sultan khan on 22 May, 2018 - 11:25 am

Dear Sir,

I have a point to clarify on Mk2 Master protection permissive "4" circuit.

In Mk2 when I checked for the condition and rating (voltage) of relays used in Permissive series circuit (3CR), I found that relays 4-1, 4-2, and 3CR relays are of 28 volt. Where as the input to this circuit is 125 V DC. With these relays fitted, the turbine was running in one station. In my experience, these relays must be of 125 V DC.

Can any one Please confirm and let me know? Now I am not in the field to practically verify. I need help to confirm this issue.


3 out of 3 members thought this post was helpful...


I don't have a set of drawings with me. But what I remember is that when the VMPA cards do their job the Master "4" relays (I think 4 of them) are connected in series across the 128V supply.

Dear Sultan

After checking the drawing of a MK2 control system controlling a frame 5 machine, I found that the 4-1, 4-2, and 3CR relays are being energized by 125V signal. However for 4-1 and 4-2 proper contacts some drives 28V and some 125V.

For example: 4-1 relay is energized by 125V. Its 1-3 normally close contact drives 125V, and it is used in the trip control circuit. on the other hand the 5-6 normally open contact drives 28V, and it is used in master four relays circuit which are connected in series across 28V.

Hope this helps, waiting for you feedback.

By sultan khan on 31 May, 2018 - 6:50 pm

Dear Mr.Isulamu...

Thank you very much for the response..

After returning to the unit, again I study the problem and found that the 4-1 and 4-2 relays comes in series with the circuit and 3CR relay is directly under 125V supply, after 4-1 and 4-2 picked up.Therefore, we can use 28 V relay for the 4-1 and 4-2 relays and for 3CR, it has to be 125 volt relay. It is practically verified.

Thanks for your interest in the participation.. and for the response...