Transformer Overload Test - Need help

EMengineer

Involved In Discussions
#1
cl. 15.5.1.3 (Overload test) says that, if first protection activates, You need to shunt it. After that you make a little smaller current, which activated protection, and then test the transformer or wait till second protection device enables. The question is: why to use smaller current? If I use smaller current than that which enabled first protection device, what is the point of smaller current? It shouldn't activate second protection device because it's value is even smaller than that current which activated first protection device.
Could someone explain this? I would appreciate for help.
P.S. First protection device is electrical protection from short circuit, second would be fuse
P.S.S. Sorry for my English (not native language), hope You'll understand :)
 
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Peter Selvey

Staff member
Moderator
#2
The second protective device is usually not relevant.

If it is, it is usually because the first protective device is current based while the second is thermal based.

For example electronic protection is set at 1.00A.

So, the test is performed at 0.99A (just below).

The transformer and switching parts slowly rise in temperature and eventually trips a thermal based device e.g. a thermistor on the heatsink for the main switching transistor, or a thermal fuse in the transformer. That would be a secondary protective device. It would stop the test.

In such a case, the secondary protective device becomes the real "protective device" and should be controlled as a safety critical part.
 

mehrnaz

Starting to get Involved
#3
Dear Mr. Selvey
I have a question regarding the short circuit test of transformer output winding. if there is a transformer which has 2 output voltage of 12 and 24 volts. according to clause 15.5.1.1:
Transformers of ME EQUIPMENT shall be protected against overheating in the event of short circuit or overload of any output winding.
Each winding is tested, in turn........
This means that there should be protection in the case of short circuit of the 12v winding at once and in the other test short circuit of 24v winding shall be tested?
Is this necessary to check the short circuit between 12 v and 24 v, which are beside each other in the output terminal of transformer?
Because if I short circuit 12v and 0 volt at once or 24v and 0 volt in another test the transformer passes the test, but by short circuiting 12v and 24v the protection does not react to this test and the temperature of output wires exceeds...

Thanks in advance...
 

Peter Selvey

Staff member
Moderator
#4
In a way this is two different subjects; windings, as an energy source that supplies secondary circuits, which need to be rigorously tested for overload and short circuit by virtue of being connected to a complex circuit that no body really wants to delve into the billions of possible fault combinations that could occur in that circuit. Just add some protection and make sure it is effective no matter what, and everyone can sleep.

Separately we the possibility that the 12V circuit can short to the 12V circuit. That's a valid concern but the fault permutations are just one (1). In that case it's worth to investigate the fault directly. For example, inspecting the creepage and clearance, dielectric strength. If these don't meet the requirements for 2MOP, short away. If the short fails the temperature limits, it's a fail result.
 
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