Functional Description and Factory Acceptance Testing

Well it has been a while since the last post and I thought we will kick off this post on the above topic. We may have touched on this in the past, but we will look at a bit more detail and how to draw up the documents.

Let’s start with the Functional Description (FD) and what it is. To keep it in plain English, it is an exact description of how the installation needs to operate. It will state the sequence of events for the electrical circuit. If we use a very simple example of a pump, a modulating valve and a level control in the tank, we can write it as follows:

The pump will start on Low Level

On low level, the modulating valve opens to 100%

At 50% level, the modulating valve closes to 50%

Should the tank level remain at 50% for longer than 3 minutes, modulating valve opens to 75% and remains there until level reaches 70%. At 70% level, it will close to 50% again

At 90% level, the modulating valve closes to 95%

At high level, valve closes 100% and pump stops

Just a word of caution, the statement above is not from an existing system. I have just put any condition in to show how we derive the two documents from the operation of the system

With the FD written, we will now create the Factory Acceptance Test Report (FAT Report)

Each one of the conditions in the FD becomes a test point on the FAT if that makes sense.

How you set your document up is dependent on the requirements from the client or yourself. It makes sense to put it into a table format.

Level Valve Position Pump Running Signature Date
Low 100 Yes    
50% 50% Yes    
50% >3 minutes 75% Yes    


Keep adding to your FAT for every condition that is to be met. Test the system, record the findings and sign off as tested. The above is a very simplified method and these could be very large documents for big projects. The point is, the methodology remains the same. Draft your FD and derive the FAT from it. Capture every test point including alarms, fault signals etc.