Loads for Unit Auxiliary Transformer

Explanation may be expected, but with an attitude like that I doubt you'll get one.

It's about time you did some of your own work.

I could not understand properly.......!!

"The down sides... you need special ground indication equipment to know when a phase has in fact gone to ground." (You mean in ungrounded delta? or a ground relay can serve the purpose?)

"You need electrical maintenance resources to track down the grounded conductors. And perhaps most of all... when a second failure happens, it is at full phase to phase potential and not phase to neutral so the fault current is greater." (This is with ungrounded delta?)

- Again, in industrial loads with major of lighting loads - why grounded star to be used - why not ungrounded delta for reliability ? (Because lighting loads require neutral point?)

Regards,

sks

You said... "The down sides... you need special ground indication equipment to know when a phase has in fact gone to ground." (You mean in ungrounded delta? or a ground relay can serve the purpose?)

Yes... the ground fault indicator serves to show when one of the phases has shorted to ground. As to a "ground relay", that means nothing to me. You need a ground fault indicator designed for ungrounded systems.

You also said... "You need electrical maintenance resources to track down the grounded conductors. And perhaps most of all... when a second failure happens, it is at full phase to phase potential and not phase to neutral so the fault current is greater." (This is with ungrounded delta?)"

When you loose a phase to ground on a ungrounded system, you must isolate the fault and repair it as soon as possible because of the danger involved in a full phase to phase fault occuring.

You also said... "- Again, in industrial loads with major of lighting loads - why grounded star to be used - why not ungrounded delta for reliability ? (Because lighting loads require neutral point?)".

Lighting loads do not require a neutral point. The benefit of using a grounded system is the reduction in fault current levels.

Oh! Yet again!

In electrical rotating machines, any earth fault would definitely involve the core. And, the core laminations have a very limited I2t capacity. Manufacturers generally give this as core damage curve. If you earth your system supplying rotating electrical machines by solid earthing or low resistance earthing or reactance earthing, the earth fault current would be much more than the tolerable levels of the rotating electrical machines connected to the system. And, core damage is sure to happen. Winding damages in rotating machine scould easily be rectified with local rewinders. But, for rectification of core damage, the machine has to be sent back to the manufacturer; thus causing an unacceptable reapir time and plant downtime. In case of unearthed system or a high-resistance earthed system, the earth fault current is kept to a value less than the core damage current levels. Thats why in systems supplying rotating electrical machines, it is preferable to have a high resistance earthed system or better still, an unearthed system, wherein the current is kept below the core damage levels, even in the case of an earth fault in the system. But, it might be difficult to identify or locate the first fault.

Then why in other commercial and industrial (lighting loads), the system is solidly earthed? Good question! You see! In commercial and lighting installations, it would be the common public (non-technical) who could handle electicity. Here, in the event of an earth fault, if isolation is not guaranteed within a very quick time, the person could get hurt, fatally too! Only in solidly earthed systems, as the earth fault current is equal to the three phase fault current, definite isolation is guaranteed, in the event of an earth fault, even with any short circuit protective device like a fuse, without any need to go for any special earth fault protective device such as ELCBs. Of course, chances of core damage in rotating electrical machines, present in such systems, is more, but, then, human life is always more precious than a core life, you know!