Transformer tripping

[Tommy Dee]

I installed a transformer for 5 smart thermostats, 1 of the thermostats trips the transformer fuse button right when I turn on the power. There is no call for heat, the c wire isn't connected at the thermostat, just the white and red and the c wire and red to the transformer. It is a 150VA transformer and I have no problem when its powering the other 4. I have a Taco SR506-2 controlling hydronic heating system.
Any help on what is causing the breaker on the transformer to trip would be greatly appreciated. I am using Functional Devices TR150VA001 Transformer.
Thanks
Tommy Dee

 

[littlespark]

Is it just the one thermostat? which would indicate that it was faulty,
or is it any combination of any 4 will work, and adding any random fifth trip it out?

 

[Tommy Dee]

Is it just the one thermostat? which would indicate that it was faulty,
or is it any combination of any 4 will work, and adding any random fifth trip it out?

Just the one thermostat will cause it to trip

 

[Avo Mk8]

It sounds like there is either something up with the wiring to (or from) that thermostat, or the thermostat itself.
If you disconnect all the thermostat 'output' wiring, just leaving transformer to thermostat, does it still trip?
Try using a multimeter to compare resistance measurements between the working circuits and the tripping one?

Added note: the transformer 'trip' is an overcurrent device, so immediate tripping indicates a severe overload, such as a short circuit or excessive load somewhere!