From: "Roger Hayter" To: <[EMAIL PROTECTED]> <snip> > I should be interested in other views. >
The following might be of interest. I claim no ability to judge the technical merits osf any of these responses, but they are all from helpful, competent folks. -------------------------------------------- <Start quoted text from another list:> Well, you're right - you shouldn't. The reason why is that surge protecton devices (SPD's) usually rely on a cheap, sacrificial device - usually a MOV (metal oxide varistor) or Zener diode connected between the hot wire and ground. The idea is, in an over current situation, the device would shunt excess current to ground. There's usually a fuse or circuit breaker in the circuit as well, in the event of a prolonged surge, or loss of the SPD. More expensive SPD's sometimes use a gas tube instead of a diode or MOV. This is to protect against sparking which might occur as the MOV or zener burns up in extreme surge conditions. The problem with connecting the devices together in a series of power bars becomes apparant if you examine the schematic of one. It's in parallell with the outlet sockets it protects. Connecting another device one of the outlet sockets connects the second device also in parallell with the first, not in series. This is not a benign situation at all. Now, you've effectively cut your surge protection in half and your only safety measure is now the fuse in the first power bar. When you connect 3 or more in a daisy chain, you might as well connect a lightening rod to your equipment. You can get very expensive SPD's which are designed to work daisy chained together. Generally speaking, you'd recognize these devices from the price. In Canada, they generally start around 150.00 or so, compared with the 6.95 for a complete power bar with surge protection. For the record, most UPS systems employ MOV or Zener diode type surge protection. Not only that, but you should never daisy chain UPS systems, which some people think they can do for redundancy. Except it's not redundant. APC has a FAQ on this at their site somewhere for those who are inclined to read up on it. <End quoted text.> --------------------------------------------------- <Start another view from a knowledgeable friend: > Typically, cheaper UPS products operate in an "off-line" fashion, generating power and switching on only when the power fails. More expensive UPS products operate continuously, actually taking wall power, rectifying to DC, and then using pulse width modulation and usually a ferroresonant transformer to re-output a sine wave (AC power). Since traditional UPS products are always rectifying/inverting, there is NO switching time, and the power is always coming from the UPS. This provides high isolation from the wall power, as transients / brownouts etc. in the wall power never get to the protected computer. Most cheaper power bars / conditioners use a combination of metal oxide varistors and zener diodes that will become conductive during a over voltage situation of adequate energy. These devices are "mujahedeen warriors", basically they typically give their life the first time they are used. Everyt time a power bar has protects a computer from a surge, it permanently degrades in its ability to protect from future problems. Since power bars do not generate a wave form, and they basically contain devices that sit ACROSS the different lines, they do affect each other when put in sequence. Power polarity, of course should be constant, and there must be a frame ground present for these things to work at all. Putting two transformer/rectifier based devices in sequence CAN potentially cause problems, especially if different sharing reference grounds. Two traditional on-line UPS in sequence, with equipment from the same computer(s) plugged in is a problem waiting to happen, as the UPS sine wave may not necessarily be in sync, generating a voltage potential of over 500 volts. <end quote> -- burns _______________________________________________ Linux-users mailing list Archives, Digests, etc at http://linux.nf/mailman/listinfo/linux-users
