Sooner or later you will hear that blue transformers can be dangerous to modern decoders. It’s not the colour that makes them dangerous.
See also:
Electricity 101 (Part 1): What does “16 V AC” Mean? What is Effective Voltage?
Electricity 101 (Part 2): What Does Nominal Voltage Mean?
Electricity 101 (Part 3): AC and Rectifiers
Modern decoders by ESU are specified for a maximum voltage of 40 V. Modern decoders by Märklin are specified for a maximum voltage of 40 V. The reversing voltage used with three-rail AC is 24 V. So there shouldn’t be any risk, should there?
With respect to the maximum voltage allowed by the decoders, we have to look at the peak voltage because there is a rectifier inside the decoder (see Part 3). We learned in Part 1 that the peak voltage is about 41 % higher than the effective voltage. So with an effective reversing voltage of only 28.28 V we already reach the 40 V limit of the decoder.
In Part 2 we learned that the voltage varies – most of the time it’s higher than the nominal voltage.
Another problem: the nominal mains voltage used to be 220 V in large parts of Europe (240 V in the UK). Now we have a compromise voltage of 230 V in most European countries. Similarly in the USA the mains voltage was raised from 110 V to 120 V.
With respect to warranty, the situation is simple: Märklin write in the instructions manual that comes with every newer loco that only transformers made for the current mains voltage may be used. This means: do not use transformers made for 110 V in the USA, do not use transformers made for 220 V or 240 V in Europe.
I have three blue transformers made for 220 V mains voltage. Two of them have a nominal reversing voltage of 23 V. With a mains voltage of nearly exactly 230 V I measured reversing voltages of almost 31 V with one of them. That’s clearly above the specified voltages of modern decoders.
Brittle Insulation
If you want to continue using your blue transformers: please note that some types suffer from deteriorated insulation material and might expose you to the full mains voltage. And only qualified technicians should open transformers to inspect the insulation.
So buying a new 6646/6647/66470 transformer will be the safest option: safe for you, safe for your decoders – at least you don’t lose factory warranty when using new locos.