Automatic Transfer Switch question - Page 1
Forum section: electronics / beginners

author: digitalnomadjc
author_rank: Contributor
timestamp: February 04, 2014, 05:32:51 am
content: Hi,I am looking at buying an automatic transfer switch for some IT equipment at a remote site. The technical specs say that it can switch to the secondary power source within 8 - 12 milliseconds. It goes on about preventing relay welding by disconnecting the original power source first, etc.This unit is only for 16 Amps. I already have a 3 phase, 26kVa ATS at my primary site for my diesel generator and it definately does not take 8-12 msec. More like 5 seconds.My question: is my servers/comms equipment going to crash if they lose power for 8-12 msec. Given I am using 230Volts AC @ 50Hz, one AC cycle is 20msec so therefore I am going to lose half a cycle? Does that matter for a switching power supply?

author: Skimask
author_rank: Super Contributor
timestamp: February 04, 2014, 05:51:50 am
content: I think the spec you might want is the power supply "hold up time", usually expressed in milliseconds, and usually not very many of those at that.You might have a power supply rated at 20ms hold up time at full load. I would think a person could be reasonably sure that, under optimal condition, it would "hold up" for 40ms at 1/2 load, etc.

author: krish2487
author_rank: Frequent Contributor
timestamp: February 04, 2014, 11:38:30 am
content: @digitalnomadjcYes any IT equipment will fail if power has been momentarily removed for >10 ms for a 50 Hz line frequency and about 8 ms for a 60Hz.The reservoir caps in any power supply, at best, are rated for half the line frequency hold up. Usually they are quite underrated.Almost any ATS is always a break-before-make type, since accidentally turning on both the static switches inside will short the two independant sources.I suspect that the 26 KVA ATS you mentioned about is only a transfer switch designed for a delayed turn on to a secondary source.I am not sure, but AFAIK generator transfer switches are (mostly) delay based transfer switches.@skimaskThe "hold up" time you are referring to is at the output of a DC PSU.The OP, i understand, is interested in the holdup time of the input AC supply.:-)

author: dfmischler
author_rank: Frequent Contributor
timestamp: February 04, 2014, 02:30:45 pm
content: My experience in this area is old, so maybe something newer is available. If you lose external utility power then your ATS will switch to your alternate source (usually a diesel or natural gas generator). And the generator will start, but will not really be ready to power your equipment until it has warmed up a bit. So the ATS should switch to the generator, and feed the UPS that will be powering your equipment for the short period while the utility power is out and the generator is not yet ready to be used. Then the generator will take over and charge the UPS batteries so it will be ready to be used again.

author: digitalnomadjc
author_rank: Contributor
timestamp: February 05, 2014, 04:13:10 am
content: Thanks everyone for your responses.The sales brochure was claiming "seamless transition" and I though "yeah, like I believe that".@krish2487You're right. I'm using AC power supplies. That was my suspicion but I wasn't sure.@dfmischlerWith my 26kVa ATS that's exactly what happens and the UPS supplies take care of the power interruption during the generator warm up.This smaller ATS was an attempt to safeguard against UPS failure at remote sites. I've had a hardwired UPS device short out and because it was a relatively large power feed I couldn't power anything up until the warranty replacement arrived which caused a significant outage. This smaller ATS would be switching UPS power and mains power with no generator.Thanks everyone

author: krish2487
author_rank: Frequent Contributor
timestamp: February 05, 2014, 05:28:15 pm
content: You might want to see if the power supplies are power factor corrected.Any power factor corrected supplies DO NOT have tank caps on the input post the rectifier stage.In such a case, the power supply will lose all power as soon as the AC mains fails.Any IT equipment power supply is switchmode, hence no tank caps even in the output (nominal but only present to smoothen ripple and not too high in case it messes with the loop stability of the PSU).

author: Waifian
author_rank: Regular Contributor
timestamp: February 05, 2014, 05:36:51 pm
content: Also, Keep in mind that generators take time to start up after power failure as well.