On 01/01/2016 11:37 AM, andy pugh wrote: > On 1 January 2016 at 17:29, Bertho Stultiens <ber...@vagrearg.org> wrote: >> The reason for /not/ connecting the ground on a secondary winding is to >> prevent a capacitively coupled ground path. > Another reason is that if you connect either secondary winding end to > ground using body parts then nothing bad happens. This is the point of > an isolation transformer. > If there is an isolation transformer fitted in the machine then > grounding one of its terminals makes it pointless. > Well, it still isolates the 120 V circuit from disturbances on the 240 V input. So, if your building ground to neutral has a bunch of noise or offsets between them, the internal 120 V circuit will be "clean" with respect to the frame ground.
The problem with a totally floating 120 V circuit is that various devices plugged in to that system may have somewhat large capacitors between hot or neutral and safety ground. Very typical in switching power supplies. Depending on the configuration of all the devices on that supply, it could cause the neutral to be higher than the hot (with respect to ground). This could cause some equipment to malfunction, if they assume the neutral will stay near ground potential. Also, a fault in one of the units could persist for some time until somebody touches something, and gets a nasty shock. Open safety grounds are a common problem in the US. I've gotten zapped quite good a few times when touching two pieces of equipment at the same time. Leaving the secondary totally floating is kind of the same thing. In the UK, they ground the center tap of those job site transformers for exactly this reason. Now, when testing inside power equipment using an isolation transformer so you can use a scope on a switching power supply, for instance, then you DO want the transformer secondary floating. Jon ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
