Make before break 1

[AusLee]

Hello,

I have these questions about "Make-Before-Break" or "late-break-early-make" arranjment of breakers (typically in ATS).

1. Does it make any sense or is it literally wrong to specify an interlocking system between the breakers. I think it is wrong because in this case one breaker must open befoer the other can closed (due to the mechanical interlocking)and so it cannot be Make-Before-Break under any circumstance.

2. Can you please point me to an example where this situation of Make-Before-Break is described or installed? i would like to know how the wiring and control are implemented to assure the coordenation.

3. I have two transformers that will never enter in parallel, thus a tie break in the MDB is used only to aupply either part of the MDB when one transformer is in service. Thus the s.c. rating of the breakers need to be only that of one transfoer, not the sum. Qustion is: in case i want the make-before-break, does the generator add too much s.c. current in order to chose another set of breakers or its participation is negligible, say the tafo is 3200A and the generator is 2500A.

Thank you.

 

[alehman]

1. It doesn't make sense to me to interlock breakers in that scenario. What you can do is put a timer in the controls to limit the interval where both are closed, if that makes sense with what you are trying to do.

2. This is normally controlled with relays which require matching voltage and phase angle between the sources. If operation is longer than a brief closed-transition period, you will probably need more complex protection systems such as directional overcurrent sensing.

3. I don't understand your system. Where is the generator connected? Paralleling a generator on the system would increase the fault current (and it could be signficant).

 

[waross]

The "late-break-early-make" that I am familiar with in transfer switches is in the poles of one switch or breaker.
That is, in a 4 pole switch, the three power poles open first and then the neutral pole opens. When the switch closes the neutral pole closes first and then the power poles close.
If this feature is needed it adds cost and the possibility of an open neutral in the event of a switch failure. We have all seen one pole of a breaker fail open. With a "late-break-early-make" we have an added level of mechanical complexity and possibly a greater probability of an open neutral.
There are protection issues with a switched neutral in a transfer switch but this can go both ways.
I prefer the solid, unswitched neutral but I don't always get to say.
Some systems are using make before break to transfer to the utility so as to not have an outage when transfering. A fast transfer may damage large motors. A transfer delay of 5 or 10 seconds is annoying to the occupannts.
As I understand the system it is a feature of the transfer switch. When the transfer switch detects the return of stable utility power it goes into synchronize mode and at the proper instant closes the line breaker to synchronize the generator with the line. Almost immediately the transfer switch opens the generator breaker.
These features are built in to the transfer switch and pre-wired. Breaker interlocking would defeat the operation of this type of transfer.

I am seeing a new type of transfer switch on the smaller gen-sets. It is an extremely fast transfer. The transfer is so fast that the panel has a synchronizing circuit similar to the closed transition or make before break ATS.
I have an installation manual beside me, but it doesn't give the transfer time. It does say that the switch will transfer when the line and generator are within 2 Hz. of each other and within 20 degrees. It does not specify a transfer time.
If you want to google it's a GE Zenith Controls, ZTX Series Automatic Transfer switch. 40-400 amps.
I hope this answers some of your questions AusLee
yours

 

[davidbeach]

I just want to add that during any operational consideration that could have two sources in parallel, regardless of duration, you should evaluate the fault currents of the parallel combination. Switching events are always a good time for other things to go wrong, and if you have two sources tied together, even for only a few cycles per month, eventually (maybe not in your life time, but eventually) there will be a fault that is fed by both sources and if the system can only interrupt the fault current from one source or the other, but not both, the smoke will be let out somewhere. And, that is not a good thing.

 

[AusLee]

I think the ZTX will not do because it has mechanical interlocking:

"The ZTX transfer switches are designed for standby applications, and incorporate a double-throw, mechanically interlocked contactor mechanism."

So if you can name another one and i will contact the manufacturer for the control drawing.

The generator is the other side of the ATS: one side is the network, one side is the generator. So if network side sc. is 50kA, should i increase the ac. rating of the circuit breaker given that with the early make the sources will be in parallel for a faction of the time?

 

[waross]

Hello AusLee
What size is your generator?

The Point of the ZTX is that it transfers so extremely fast that you get the advantages of closed transition with an open transition switch. I am in the process of installing one and in a week or so I will have first hand experience.

The present installation is a customer purchased set from a company in Florida that i am not ready to recommend until a few issues are resolved.

Most of the sets that I install are supplied by these people. I have not bought a ZTX from them.
Wilkinson & Gaviller Ltd
High Street
Staplehurst
Kent
TN12 0AR
WGLLTD@aol.com
Tel: 01580 893 514
Fax: 01580 893 525
My contact is Mr. Godfrey Hooper

Manufacturers; Try F.G. Wilson in Ireland. This firm was purchased by Caterpilar several years ago. Their sets are available under Various Trade Names. They manufacture the Olympian line that is marketed by Catepilar.
yours

 

[alehman]

GE makes a line of closed transition automatic transfer switches, if that's what you're looking for (try model #ZTSCT). These have < 100ms overlap on all poles.

Short circuit rating for this situation is a common question with no pat answer. To be conservative, consider the combined rating. The logic being that a failure during switching is a possible cause of a fault.

 

[powerwagon75]

AusLee,

Don't know if this thread is already "dead" or not, but you asked for MFR examples--Cummins (Onan) makes a Breaker combination switch-- in hard-load or soft-load transition modes. Uses a very well built controller, and when coupled with their own genset, can be connected for communications and will add the generator warnings to the scenario to decide if to go closed or open transition. There are other manufacturers as well, and local companies who build them with PLC logic.

I'm guessing you're in the market for a larger switch, but check the link out. Their new OHPC (up to 800 amps) switch uses a unique "blow on" technology on the contacts, where fault current increases the contact pressure. It's a high end price tag, but nice product. Saw the video of it going through the short circuit testing; Switch just jumped/wires danced, and that was it. Same test, brand "A" switch, and the orange globs of molten metal were bouncing off the ceiling of the test cell. That switch was history after that.

http://www.cumminspower.com/library/salesliterature/Americas/TSControlsBrochure030105.pdf

Here's a link to their hard/soft transition switches:

Hard:

http://www.cumminspower.com/Commercial1/Switchgear/S-1349.pdf

Soft:

http://www.cumminspower.com/Commercial1/Switchgear/S-1348.pdf

I apologize if it sounds like a sales ad--it isn't. Just a little biased as their products have given me the least amount of headaches in my 25 years.