Surge Passing Through a Double Conversion Type UPS

A3: fit lightning protection to the premises according to current local standards. It will not prevent absolutely any damage, though it will minimise the risk.

Thaks for your reply.

Does it sense that, theere is a possibility in passing a surge hrough double conv. online UPS??????

Thanks!

Others may answer the other two questions. Be patient!

A lightning surge can be present on one or both conductors to the UPS. Unless it has an internal short to earth, it should not be affected when the lightning is present on both conductors.

If the lightning is present on just one conductor it is possible to disable the UPS entirely. So, a good surge protector that uses metal oxide varistors helps as long as they are located very close to the conductors providing power. In other words, do not hang them as an accessory on some long wire.

The more MOV's the better. Each one is capable of a couple thousand Ampers for a short time (as in a lightning strike). Several in parallel provides extra capacity and assurance that some have survived after the incident. Don't worry about the number of joules they advertise for capacity. Instead, look at the total peak amps they can bypass and at what level they kick in. A protection level on 20,000 Amps peak is a good starting point.

1/Yes

2/Yes. If the UPS has neutral bonded through and the UPS is therefore not a separately derived service, then definitely. However, even UPS' configured as separate services would be susceptible through the transformers and the grounding conductor (potential rise).

3/Joe has posted a good idea. Here is a good paper from Littelfuse on the subject.

Yeah, all the guys replied something useful, but.....

The double conversion UPS - AC-DC-AC - is conceived to prevent this and similar problems. I had a few - well designed ones - operating over decades without fail.

BUT, both design and maintenance have to know, and take into account, at least of the basics. If you place the batteries - for structural reasons on the other side of the building, how much of inductance is introduced, that nobody asked for? Maintenance did maintain the low ohm connections?

Unless such questions are handled properly, lesser issues are of no importance.

Hopefully all of your DC cables are properly shielded through properly bonded conduit, shortest runs feasible, routing, etc. and those circuits would therefore not be able to introduce the effects of a lightning strike through the inverter portion of the UPS.

While your opinion is valid, That is emphatically not the point. Original designs assume, for example no induction in the leads to the battery, not any induction in the construction of the batteries themselves.

On the other hand, dI/dt limits the dampening effect a battery set can exert. Long cable = poor dampening. That's it.

I thought the OP's question was about switching/lightning surges to the inverter portion of the UPS:

1/ Is there any chance a Switching/ lightning surge occured in the upstream of UPS, passing upto the load via an on line double conversion type (transformer less) UPS.

2/ Some suppliers says, in this ups the neutral and earth is internally not disconnected, and therefore this can happen. is it true???.

What effect would DC induction have on the neutral of the supply other than the obvious overall loading and inefficiency induced by long cables?

I didn't know we had switched to induction theories from DC.