MX-60 operating voltage limit [RE-wrenches]

[Christopher Freitas --- OutBack Power]

The MX60 is ETL listed for a maximum PV array open circuit voltage of 
150VDC "worse case" - i.e. under the coldest temperatures.  That said, 
there is still a reasonable amount of safety margin left in case the 
voltage does exceed this 150 vdc limit - I am running a MX60 on my own 
system with seven seimens m55 modules in series (a nominal 84vdc PV 
array) too simulate a 72vdc PV array in very cold climates - and it has 
survived for almost five months now.  We do not recommend doing this 
however!   

The FETS are rated at 150 vdc but don't fail until about 175 or so.  The 
capacitors are 160 vdc rated - but can tolerate maybe 185 or so.  The 
real limit is the circuit breakers and GFP right now - we have an 
additional ETL listing on them for 150VDC for PV applications 
specifically - the UL listing for them is 125VDC.  If we increase the 
rating on the MX60 then we need to increase the rating on the circuit 
breakers / GFPs as well of course.      

The MX60 will go into a self-protect mode at 140 to 142 and will wait to 
start up until the open circuit voltage of the PV array drops below this 
level.  This happens fairly quickly once real sunlight hits the  PV 
array and the cells warm up.  This delay in start-up does not seem to 
cause any significant power production loss as the highest open curcuit 
voltages seem to be during the early morning when there is indirect / 
ambient light only on the PV array.  

There are older MX60s which had the ETL approval at 140 or even 135VDC - 
maybe this is what the tech was mentioning.  We can upgrade them to the 
newer specification if you need it.  There are postings on our OutBack 
forum which discuss the serial numbers involved if you want to see all 
the details of when and what features were added.  

I know of many MX60s used with 72VDC PV arrays in cold climates 
(Montana, Colorado, ETC) which have been working for years and 
occasionally hit open circuit voltages in the mid 150's - so I would not 
be too concerned about the MX60 itself.  

The only real issue is what do you tell the inspector who just was 
trained on PV array cold temperature multipliers in the NEC...   


Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  


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