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Tech Help - How does a redundant PSU work and what are the benefits?

Published by Richard Parry on 29th Apr 2019

A redundant PSU is a power supply that helps prevent your server (or workstation) from going down in the event of a PSU failure.

So, what is a redundant PSU?

Commonly this is made up of a PSU Cage, which in some instances looks very similar to a normal single PSU. It is a similar size/shape and out the back has exactly the same connections you expect – it has power connectors for your motherboard and for connected devices such as HDD’s (Molex and SATA). Some redundant PSUs also have PCIe graphics card power connectors, although this is less common as it’s rare for GPUs to be used in a server.

Most redundant PSUs also have a cable with a little connector called PMBUS. This will connect to your motherboard (usually only server motherboards though) and allow your board to read information about the PSU, such as temperature, fan speed, voltages and most importantly, the health of the modules.

Within the PSU Cage are at least 2 PSU modules. Each of these is in effect a full working PSU, however it will be designed for hot-swap, which means you can remove this module from the server without the loss off power or any effect on the system whatsoever.


How is it redundant?

By having 2 or more PSU modules, this means that if one module fails there is always a second (or sometimes even third) module to keep things going. There will be no effect to the server at all if a PSU module fails. When in normal operation each PSU module will “load balance”.

So if you have a dual redundant PSU, which has 2 modules, and the PSU is rated 500W, then under normal operation it will balance 250W (Max) load across both modules, however each module will actually be 500W rated – so if a module fails then the remainder working module will take the existing load in full.


So why a redundant PSU?

Redundant PSUs are more expensive than a single PSU, sometimes well over three times the price depending on the model and its output wattage. However, it is all about uptime and risk management.

You have to ask yourself the honest question – If this server goes down due to the PSU failing, how much will this downtime cost? Remember you need to factor in the purchase of a new PSU, the lead-time for getting this PSU (it may go down on a Friday and you won’t get it until a Monday or Tuesday) and also the amount of effort needed to remove the whole failed PSU from the system and install a replacement. All of this takes time – What if this system was your domain main office server – the system being down means none of the staff can do any more, they can’t process orders, they can’t fulfil customer requests – you have to send your staff home as there is nothing for them to do – This whole process could run into many tens of thousands of pounds. The cost of a redundant PSU is typically around £200 more than that of a single PSU – But it gives you complete peace of mind.


Can I get one for my system?

Absolutely – Most chassis can accept a redundant PSU, replacing the existing single model. We have a huge catalogue of redundant PSUs from stock here; https://www.servercase.co.uk/shop/server-cases/power-supplies/redundant-power-supplies/

We have models that fit standard 4U cases, some proprietary models as well as some designed to go into a standard ATX PC tower case, so you could easily upgrade and retrofit your existing server.


I need help choosing.

No problem – We’re here to help – Just get in touch with us https://www.servercase.co.uk/contact-us/ and our technical team would be happy to recommend a suitable PSU either for a new server or to upgrade an existing one.

Tech Help - How does a redundant PSU work and what are the benefits?

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