From pages 67-68 of The DCS O Gauge Companion 2nd Edition:
Phasing Transformers
All transformers are plugged into AC electrical outlets that have either two or three prongs. The vertical slit openings are for AC Hot and Common while the round opening, if present, is for earth ground. The following discussion relates to the Hot and Common openings.
Modern transformers have AC plugs where one blade is slightly broader than the other. This allows the AC plug to be inserted into the outlet in only one way. However, not all transformers were manufactured with the blades in the correct orientation. Postwar transformers generally lack this blade orientation altogether, allowing them to be plugged into an AC outlet in two ways. Regardless of which transformers you use with your layout, if you use more than one transformer it's important that they all be in phase with each other to eliminate the possibility of inadvertently creating dangerously high voltages. This can occur when locomotives or passenger cars with dual pickup rollers bridge two sections of track powered by different transformers, or when a transformer other than a track power transformer is used to power switch motors wired for non-derailing operation.
To ensure that multiple transformers are in phase:
1. Start by plugging any two transformers into AC outlets of the same power strip or the same wall outlet
2. Turn on both transformers and set the output of each transformer to zero
3. Connect a wire from any Common terminal of one transformer to any Common terminal of the other transformer
4. Connect two wires, one to a Hot terminal of one transformer and the other to a Hot terminal of the other transformer
5. Set both transformers to as close to the same voltage as possible, at or above 10 volts
6. Select one of the following three ways to now test to see if the two transformers are in phase or not:
• Briefly touch the wires connected to the two Hot terminals to each other. If a spark results the transformers are not in phase. If there is no spark, they are in phase
• Using a voltmeter, measure the AC voltage between the two wires connected to the Hot terminals. If it is approximately twice the value of the voltage to which the transformers are set, they are not in phase. If it's less than a few volts, they are in phase
• Connect an 18 volt light bulb between the two wires connected to the Hot terminals of the two transformers. If the bulb glows brightly the transformers are not in phase. If the bulb is dim or does not light at all, the transformers are in phase.
7. If the transformers are not in phase, reverse the AC plug of one of them in the outlet and repeat the test. This may be difficult if the transformer has a plug where one blade is broader than the other
8. Now, repeat the above procedure using one of the two transformers that are now in phase (either one is fine) and the next transformer that will be used on the layout. In step 7, if the two transformers are not in phase, reverse the plug of the transformer just added to the test, not the one that was previously tested
9. Continue the above process until all transformers to be used on the layout are in phase.
Once the transformers are in phase, mark the AC plugs of all tested transformers so that you'll always know the correct orientation for plugging them into AC outlets.
This and a whole lot more is all in MTH’s “The DCS O Gauge Companion 2nd Edition", available for purchase as an eBook or a printed book at MTH's web store!
