Functional Testing v Principles testing, Verification and Validation

[PJW]

I was asked privately to explain the difference between Principles Testing and Functional testing. The question was asked in the context of Australia.

Thought it worth copying my answer here as I expect this may be helpful to others. Also I have never even been to Australia can only assume that it follows current UK terminology, so perhaps someone else can comment from that perspective.

I just got a doubt what is the difference between functional testing and Principle testing. Can you please tell me and what is this verification and validation testing.

I can tell you what it now means in a UK context; I guess that Australia would be the same, HOWEVER I should also tell you that when I started of the Western Region of British Rail in 1981 then what we THEN called "functional testing" is precisely what NOW is called "Principles testing"! Hence beware- usage can vary....

Functional testing is testing that checks out individual parts of a system, involving the application of power to exercise the circuits etc. So getting points to throw in response to a call, measuring that the voltages and currents are correct, making sure that the clutch slips appropriately and that the timer cuts off after 7.5seconds is an example. Similarly the fact that the point contactor relays operate in response to the interlocking and have the correct voltage and polarity and use the correct cable cores fed from the specified fuse.

So is the circuit testing in the interlocking that proves contact A1/A2 of AE TPR is a front contact and is effective at cutting the feed to 361GR so that the signal goes back to danger. The key thing is that each test done is ticked off or otherwise recorded on the design details another copy of which was used during installation to produce the product being tested. It is VERIFICATION that we have built correctly what the designer designed. "Did we built it right"


Principles testing is about VALIDATION; is what we have built "fit for purpose"? "Did we build the right thing?"
So the tester looking only at the Signalling Plan and signalbox diagram DECIDES FOR THEMSELVES what locking would be appropriate in order that a set of points should be allowed to move or a route to be set or for a signal to show a particular aspect. They then "functionally" investigate to see whether the actual implementation agrees with their expectation and only when satisfied then asserts that the locking exists. They are NOT LOOKING AT THE CONTROL TABLES, but call out to someone else who is looking so that it is then recorded by ticking entry on the Control Tables. The tester calls out the locking that they expected and had found to be effective.

Hence if the designer has made an error and omitted a control, the Principles tester will discover the error because he is not relying upon the designer (as has made up their own mind first); The functional tester however won't have discovered the error as what they have proved is that the installer has correctly wired the circuit as designed, the relay is of the correct spec, there is enough voltage for the relay to pick and then no residual voltage when it should drop.

Principles testing (at least in theory) is undertaken on the completed entire system whereas the functional occurs little by little on increasingly larger assemblies (e.g.test point machine in factory, test prewired location odff-site,, test from location to point machine locally, test relay room interlocking circuits, test between interlocking and location, test panel circuits, test between panel and interlocking etc.

In practice, particularly with Solid State Interlocking some of the distinction (in terms of which grage of staff do what) is a bit blurred; rehearing the operation of points from the control centre to the point machine is technically Verification rather than involving any Validation, yet generaly the activity is led by a "Principles tester", oartly because there may be a need on the same shift to do some Principles, but often just because they are the more experienced people who can
a) get things to work when there are problems and others would flounder, and
b) just might see something very important that the less experienced would miss.

Hope that helps.

PJW

[greatnessjason]

I was asked privately to explain the difference between Principles Testing and Functional testing. The question was asked in the context of Australia.

Hi PJW,
In Mod3 2012 Q2 the question is
Prior to commissioning, some railways verify that a signalling interlocking is operating in
accordance with design. Other railways undertake testing against the fundamental principles
and attempt to “break” the logic. Discuss the advantages and disadvantages of each approach
and the reliance of each approach on control tables and the design process.

Is it true to say what you have described above that functional testing is testing to "verify that a signalling interlocking is operating in accordance with design" ( in this case control table design) and principals testing is "Testing against the fundamental principles and attempt to 'break' the logic"?

[PJW]

Broadly.
Certainly uk Principles testing does fit: "undertake testing against the fundamental principles and attempt to “break” the logic".
My interpretation of the "verify that a signalling interlocking is operating in
accordance with design" is more meaning a "systems engineering" approach of having formally defined requirements and traceability that the system delivers these- often uses automated testing to demonstrate that the specified rules are achieved. This I think s more common in places such as France and Germany.

I was asked privately to explain the difference between Principles Testing and Functional testing. The question was asked in the context of Australia.

Hi PJW,
In Mod3 2012 Q2 the question is
Prior to commissioning, some railways verify that a signalling interlocking is operating in
accordance with design. Other railways undertake testing against the fundamental principles
and attempt to “break” the logic. Discuss the advantages and disadvantages of each approach
and the reliance of each approach on control tables and the design process.

Is it true to say what you have described above that functional testing is testing to "verify that a signalling interlocking is operating in accordance with design" ( in this case control table design) and principals testing is "Testing against the fundamental principles and attempt to 'break' the logic"?