Following several hours running on a backup generator, our mast in Merstone has not returned to running on mains. We recently upgraded the backup battery system in this mast, it has now been identified that there is a minor design fault - resulting in an outage it in the network this evening. We are sorry for the disruption to service – the design fault will be rectified in the next day or so.
For those of you slightly techy or just interested read below: -
The equipment running in our mast sites is low voltage DC – the site in question is a 24 volts (nominal) DC.
Essentially what all our sites have is a DC rectifier stepping 230VAC down to a stable 28.5volts DC which is fed into a DC UPS. The DC UPS floats the backup batteries and supplies the DC Current to the load (our kit, the kit that supplies your service).
The MAIN Fuses on this site are 10AMPS which at 28.5v were doing the job quite nicely.
280watt load @ 28.5volts = 9.8 Amps.
This evening our site suffered a mains failure (this is pretty normal this time of year) the load was automatically transferred over to the battery bank – our proactive monitoring alerted us, but with a 20 hour run time on battery this is classed as an ‘information’ alert. We need to be aware that the site is running on battery, but everything is in hand the service is not at risk – so you can imagine our surprise 45 minutes later when the mast shut down.
Now for the theory.
As batteries discharge the voltage decreases, and when the voltage decreases the amperage increases. This is known as Ohm’s law, more on that here.
The open circuit voltage of our batteries at 100% Charge is 25.48 Volts.
280watt load @ 25.48volts = 11 Amps.
The open circuit voltage of our batteries at 80% Charge is 24.2 Volts.
280watt load @ 24.2volts = 11.7 Amps.
At which point our 10AMP main fuses just could not take it anymore!!
The power system on this mast site is ‘the standard’ across our wireless network. This site however has become a bit of an aggregation point for several masts and therefore has several high capacity radio links on which do draw quite a bit power compared to other mast in the network.
So the root cause of this incident is that the load calculations did not take into consideration the increased amperage during discharge conditions on the battery bank.