On 11/11/2011 9:37 AM, Dave wrote: > On 11/11/2011 7:11 AM, Brian May wrote: >> What is meant by "bonded to the ground"? Does that mean connecting >> the nuetral leg of the transformer to the ground? If so, why use the >> transformer at all when i can just go from a leg to ground? > > If the 3 phase service is fed from a delta connected transformer there > may not be a neutral... That was very common in older plants. > Now most newer plants use delta-wye transformers so there is a neutral > connection at the center of the wye and that point is "bonded" or > connected to ground at the tranformer. At that point a real ground > is also established via ground rods. > > You asked how do more 3 phase machines derive 120 VAC from a 3 phase > input, so I was trying to answer that question. > > If you look at the bus duct systems used in most plants in the US, > most of them only have 3 bus bars - no neutral is available. > > A protective/safety ground wire is assumed. Protective ground > connections are always made to machines. Generally the first stop > for a protective ground in a power or control cabinet is ground bus > bar mounted in the corner of the steel cabinet. And the steel cabinet > is normally connected to the machine frame. > > The protective/safety ground is never used to transmit power. Using > it for a power return path is a huge no-no. If you need a neutral, > you need to run another wire otherwise you are defeating the purpose > of the protective/safety ground. > > >>>> why use the transformer at all when i can just go from a leg to >>>> ground? > > If you think about that statement for a moment; you will realize that > you just suggested that you connect the power "hot" line to the frame > of the machine via the load. > A very bad idea. > > > Dave > > When you put a transformer in a cabinet to create a 120 VAC power source then you have a separately derived system. It is harder to understand why you need to do this, than to just do it.
The wiring is very simple. You just need to do it correctly. Here is a web article that points to section 250 in the NEC (National Electric Code) ..... http://www.mikeholt.com/technical.php?id=grounding/unformatted/Separatesystems&type=u&title=Separately%20Derived%20Systems%20%5BTransformers,%20Generators,%20etc.%5D%20%284-10-2K%29 Even more on neutral to ground bonding.. http://www.iaei.org/magazine/2005/07/grounding-separately-derived-systems/ See the connections between the neutral and the ground on the secondary side of the transformer? It is really that simple. Dave ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
