Technical information about AP7721

[tomk67_apc]

This was originally posted on APC forums on 4/21/2015

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Hello everbody,

i got a brand new AP7721 and I have a technical question about it.

Last week we had a power down on one of our two powerbars in our rack. Reason for this power down was a defective Netgear switch. The switch produced a short curcuit and kicks the overload fuse for this powerbar. The real problem was, that we had our CISCO ASA Firewall conected to this powerbar and  so our webservices was offline. To prevent us from having this issue again, a technician in our facility told us to buy a ATS so that the firewall with only one powersupply can be connected to both powerbars.

Now the Question: What happen if one of the devices powered by one of the outlets produces such a short circuit again? How does the AP7721 react to this? As far as i understand if the short will take down our source A and the AP7721 will switch to source B it will take down the source B too, or will the AP7721 shut down all outlets? Will the AP7721 shut down all powerbars in the rack in the worst case of a short circuit in one device connected to the AP7721?

I have not found information about that in the documentation provided on the apc website.

I hope anybody knows that otherwise I cannot understand how the AP7721 can make my rack secure against powerloss.

Thanks in advance, Tom

[BillP · Accepted solution]

This reply was originally posted by Angela on APC forums on 4/21/2015

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Hello,

As long as the AP7721 ATS is fed input power from Source A or B, the outlets will be on and providing output power. AP7721 specifically does not have internal breakers so the scenario you'd likely see will be that the primary/active source's upstream breaker will trip when a piece of load causes a short circuit, it will switch to the secondary source, and trip that upstream breaker. So, anything else connected to these two circuits feeding the ATS will come down.

Some of our larger Rack ATSs, mostly used in North America (which have NEMA type outlets), have breakers internally since the units. With one of those, you'd see the internal breakers trip on that unit. If you're outside of North America though, those units are a higher voltage and thus have a lower current rating which doesn't require breakers by applicable safety standards/certification authorities. 

Hopefully this is a rare situation in general. I am not sure if some (load) devices have some type of internal protection against this?

[tomk67_apc]

This was originally posted on APC forums on 4/21/2015

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Angela,

thank you for your fast answer. So the AP7721 is not really a helpful device in my scenario with only 2 powersources. The danger loosing the whole power in the rack is bigger, than the benefit from distributing these two powersources to the single powered devices.

Maybe I have to talk to an electrican - may we can install a fast breaker between the AP7721 and the powerbar so that we can prevent the breaker in the powerbar to trip.

Yes I found devices that have internal protection against this.

Thank you, Tom

[BillP]

This reply was originally posted by Angela on APC forums on 4/21/2015

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No problem. I'd be interested to see what an electrician recommends to you because this made me think that in the rare occurance you do have a short, you'd take everything down. If these were separate circuits from one larger source (like a giant UPS with modular PDU breakers that we offer), I believe the resolution there is a giant 3 phase Automatic Transfer Switch versus one of these little rack ones. Don't quote me on that though, just going by what I've learned on my own since I don't have much experience with 3 phase power.