Comments on: UPS protection

By: roniehalrone

roniehalrone — Fri, 13 Aug 2010 10:19:11 +0000

UPS protection is always needed to everyone who all are using computer whether it is for low or for higher purpose. UPS provides you electricity when the light is gone so that you can your computer is protected. Borri UPS Systems Blog

By: philip

philip — Sat, 07 Aug 2010 23:03:08 +0000

A UPS protection system may interfere with the Uninteruptable part of the system.
If you have batteries that are short circuit then the system will notice I am sure so it possilble that it will be a very rare event.

By: Eddy

Eddy — Tue, 29 Jun 2010 06:36:26 +0000

Always use automatic circuitbreakers otherwise you won’t be able to reach selectivity (slow overload protection when fuses are used). You also need spare fuses…

By: Frank

Frank — Mon, 28 Jun 2010 13:54:48 +0000

What type of breaker is recommended for feeding a UPS …automatic or non-automatic?

By: Mathieu GUILLOT

Mathieu GUILLOT — Tue, 02 Feb 2010 09:34:05 +0000

To be noted: Some UPS manufactuer propose dynamic power storage instead of battery with higher short-circuit current when running without supply. The following concerns only battery storage.
As KMH said: “In case the back up source is absent load will defintely disturbed by a short-circuit or ground fault. As there is no guaranty that the short circuit will be cleared within a short time.” Critical load like computer won’t withstand voltage drop longer than 20ms. (See CBMA curve) So if discrimination (selectivity) is required, maximum fault clearance time should be checked as well.

In critical application with UPS, operation on battery only is not supposed to last more than few seconds: time to switch to a back up supply or to start a generator. So the probability of short-circuit or ground fault during this period of time is very low. I have seen in one “data center” a rule for operators that forbid any works inside computer room when UPS is running on battery only.

In my opinion the protection of persons have to be checked first. As a general rule the maximum feeder downstream UPS shall not exceed 0,5x rated current of UPS. Lower value are better… The use of RCD could be necessary according to earthing system and installation rules. A technical paper from Schneider deal with this subject “CT129 Uninterruptible static power supplies and the protection of persons”.

By: Matti Sinisalo

Matti Sinisalo — Thu, 28 Jan 2010 10:53:30 +0000

Thanks a lot to everyone who shared their experience in this topic. The over-dimensioning of UPS without increasing unnecessarily battery capacity sounds an interesting solution. In my field of work I have found this B6 or B10 if group wire is very short to be maximum MCB size in most cases.

As Aad suggested that dimensioning needs to be done according to maintenance bypass values (higher short circuits). This can be avoided by installing a transformer between mains network and UPS.

By: Sirawat P.

Sirawat P. — Wed, 27 Jan 2010 22:50:16 +0000

To get proper discrimination while UPS is in back up mode we have to consider current limitation capability of inverter module i.e. 260% of nominal current for 150ms.

With above information, you can match it with your CB range characteristic.

This information is available from UPS supplier but you have to ask for it and it is varied from brand to brand as well. Some brand even recommend the maximum rating of CB with proper trip unit to be used at UPS output distribution.

By: Eddy

Eddy — Wed, 27 Jan 2010 12:37:16 +0000

To protect installations behind a ups, it’s a good idea to over-dimension the ups without increasing the capacity of the battery’s. This increases the available shortcurrent, isn’t that expensive (in the range >10kVA) and in combination with cables with larger sections (low impedances) the short-circuit current wil be as high as possible. Curve B MCB’s, small caliber as near by the load as possible (4-6 amps max) and NO further protection between UPS and load (cable section in function of max thermal load offcourse). Before UPS, protection of UPS and cables prefer choosing mesh type TN-S
In short: overdimension of UPS and cables (factor depending on demanded power)

By: ABDUL KAREEM C

ABDUL KAREEM C — Wed, 27 Jan 2010 06:52:31 +0000

In the case of interruption free online UPS systems, irrespective of kva rating (transfer time from UPS to BYPASS and vise versa less than that of half cycle period – 10ms in 50Hz supply) transfer possible only through electronic switching devices. Hence dealing short circuit currents leads to total failure. This case is addressed by turning off the Inverters as well as Static by pass and by transferring the entire load to normal bypass where the transfer is achieved with interruption in out put supply. All these will happen before the circuit breakers acts tripping.

By: Dikran

Dikran — Tue, 26 Jan 2010 16:58:40 +0000

Yes,
What Ingvar says is right, for huge UPSs 60KVA Curve C 16A breakers work, as the short circuit rating is higher. For UPS in order of 10KVA, smaller rating breakers with Curve B will be required.
I want to give you some tips, as the discussion is becoming hot:
Short circuit at the exit of the UPS =2.3xInominal
Short circuit level at bypass position = 19xInominal for a period of time, during which the breaker must trip to clear the fault. If not (short circuit still) the bypass will open.
As a rule of thumb.