Cardew

[electrolitic]

On LV isolated network Schneider recommends a surge limiter (Cardew C) to discharge overvoltages to earth, normally connected between neutral and earth.
In case of unavailable neutral the catalog shows just one surge limiter connected between one of the three phases
and earth. Is this enough or it would be necessary 03 Cardews ?
Thanks.

 

[melspuds]

Without knowing all of the details of you system, I would say yes to using one per phase and ground (earth). Make sure you use the correct voltage rating from phase to ground for your Surge Protective Device (SPD).
Hope this helps and good luck.
Dave

 

[MatthewDB]

A Cardew is a one shot over-voltage device. When it's rating is exceeded, it fails short.

Its purpose is to turn a normally floating system to a grounded system as a result of an over-voltage. Ungrounded systems have two major risks: restriking faults and transformer primary to secondary fault. (Google restriking ground faults if you're unfamiliar.)

Since the intentionally fail short, you don't want three of them. One will turn a floating system into corner grounded. Three would turn it into a bolted two or three phase fault.

 

[MatthewDB]

If you're unfamiliar with IT grounding, this is a great white paper:

http://www.schneider-electric.com/documents/designers/top-downloads/090197c6800bcbde_.pdf

 

[electrolitic]

Thanks Matthew

The things I know:
1.Cardew C is not a one shot over-voltage device. From the paper from Schneider you had mentioned:"Impulse over-voltages generates arcing but do not short-circuit the limiter. Energetic overvoltages does"
2.The Cardew C catalog describes:"It clips weak overvoltages" and "If the permanent insulation indicates a sustained fault, checks the Cardew C has not sparked over definitely. In this case the cartridge must be replaced as soon as possible". I check it automatically because I normally pass the cable that grounds the Cardew C through a toroidal CT connected to one of channels of XD312. This permits the location of permanent ground fault detecting the 2,5Hz injected by the insulation monitor IM400.
3.Avery good paper about overvoltages at IT systems is "Transient overvoltages on ungrounded systems from intermittent ground faults" by EATON. There, the overvoltages appears between phase and ground.
So, the question remains: Do I need one Cardew C or three Cardew C as Schneider does on MV ungrounded systems designs installing three L.A. (lightining arresters)?

Thanks again.

 

[melspuds]

electrolitic

I do not know were you are located, but Schneider USA has LV 3 phase ungrounded delta surge protection devices for at least 5+ years. The voltage range from 600+ to 120 volts. EMI/RFI filtered, up to 200kA SCCR, they do not take off line or or ground your system. This type absorbs the surges and can be installed any place in the system.
google: Square D
Hope this helps, and good luck!
Dave

 

[melspuds]

I just checked, Eaton us has them also. Up to 400kA/phase.
Dave

 

[electrolitic]

Thanks melspuds
Could you please send me the links?

 

[MatthewDB]

For weak overvoltages it will clip. For strong overvoltages it intentionally becomes a solid short.

You can download the manual here:

http://www.schneider-electric.com/download/ww/en/details/2630573-Cardew-C---Surge-limiter/

It shows only one phase connected.

 

[electrolitic]

Thanks MatthewDB

The old and superb Donald Beeman handbook (1955) so as the mentioned EATON white paper describe dangerous phase to ground transient overvoltages on IT systems. My final question: "An IT 480V system, wye secondary windings connected, unvailable neutral, one surge limiter connected just at one phase to ground. Is my system protected against overvoltages between the other phases to ground?"