What is NH overspeed test and how it is preformed on DHC Q400 aircraft? In the Aeroplane Operating Manual of DHC Q400 aircraft it is written that, at the time of normal engine shutdown, the engine system tests the NH overspeed circuitry by using it to shutdown the engine. 


I didn’t understand that, how the NH O/S circuit is used to shutdown the engine and how it is tested?
And what are the conditions that will ensure the correct operation of NH O/S circuit?
The same statement is made here in a document describing the PW150 turboprop engine, which powers the de Haviland Canada, now Bombardier,  Dash-8. 

It gives a little more detail in section 12.23.14.4, NH overspeed protection.

It says:


  The PW150A Powerplant has an independent overspeed (O/S) protection circuitry (dual chan- nel) built into the FADEC which has the capability to cut off the fuel flow through the Fuel Shutoff Solenoid. Independent NH signals (from the FADEC) are used by the O/S circuitry. A fuel shutoff command is issued when the measured frequency of the NH input signals exceed a pre-pro- grammed threshold value of 108%. The O/S Protection circuitry is exercised on normal shut- downs by the FADEC.


So, it would appear that, when testing the overspeed protection in a normal shutdown, they do not actually allow the shaft speed to increase to 108%, but rather they electronically alter the speed sensor frequency signal, so it appears like the shaft speed is greater than 108%, which then cuts off the fuel flow.

NH is the term used to describe the high pressure shaft speed, measured in units such RPM.