[RE-wrenches] Third Radian ---> Class T Fuse With/Without Fuse Holder

[Matthew Sirum]

Greetings William,

Good point.  This is something I always take into consideration.  I
have used Class T Fuses in many situations with and without their fuse
holders.  In the case of the installation shown in my custom DC Bus
System pictures the wire/cable type and wire management avoid putting
too much physical stress on the Class T Fuses.  The #4 AWG conductors
are supported from above and apply very little stress on the fuses
they connect to.  The 4/0 AWG cabling is Cobra X-Flex (fine stand,
very flexible) and each of these conductors is secured a few inches
from where they connect to their fuses.  Care was taken in making up
these connection and they were all carefully adjusted and inspected to
ensure they would not stress the fuses.  I feel really good about how
it all came together.  The same care and attention will be needed
whenever these connections are serviced and the fasteners are
torqued/tightened.  When well planned and carefully assembled I do not
have concern there will be any issues related to not using the fuse
holder.  I also think about this another way as well...  Using the
Class T Fuse Holder adds another 2 connections to the circuit.  I try
to remind myself that more connections can equal more chance of
issues...  Although, if a manufacturer of a Class T Fuse states the
fuse holder must be used with their fuse there could be an NEC
violation by not using it.

One thing to note about the use of these fuses in this installation is
that they are supplemental over current protection (OCP).  Each
positive (ungrounded) conductor connecting to this DC Bus System has a
means of primary over current protection and disconnect on its
opposite end.  The battery systems, the charge controllers, and the
inverter/chargers all have circuit breaker disconnects on their
respective positive circuit conductors connecting to this DC Bus
System.  I have seen other folks build without having the needed
appropriate means of disconnect and OCP; although most folk are
probably more likely to include the primary OCP, but not consider the
supplemental OCP.  The supplemental OCP (Class T Fuses) provide
protection in the event of a fault or short in a circuit between the
Bus System and the other OCP at the other equipment.

Like many folks I have installed and worked on battery systems where
we install a Class T Fuse directly onto the positive terminal of the
battery bank or via some form of busbar or connector etc.  In these
cases not only do we need to take care not to stress the fuse, but the
fuse itself (and added busbar/connector etc) can act like a lever and
along with the mating cable lug(s) create too much stress on the
battery terminal...  Potentially damaging the terminal, the inner
battery connection to the terminal, and/or the seal between the
battery case and the terminal.  Carelessly tightening the
hardware/fasteners to assemble these items to the battery terminal is
a common way of inflicting damage on the battery as well.  I know I
have been involved in some service situations where there was more
stress on the Class T Fuse than I would like to see, but as best I
know there have not been any fuse failures because of it.

Thanks for bringing up this question!

Best regards,

----MATT

Matthew Sirum
P.O. Box 1227
Greenfield, MA 01302-1227  USA
phone: +1.413.773.0611
email: MatthewSirum at gmail.com


-------- Original Message Below --------

[RE-wrenches] Third Radian

William Miller william at millersolar.com
Tue Apr 25 23:03:03 PDT 2017

Matt:

I have a question for you:  I have often considered mounting Class T fuses
directly to a bus bar as you have done.  Are you ever worried about physical
strain on the fuse itself without a fuse holder?

Thanks again for all of your input.

William Miller

Lic 773985
millersolar.com
805-438-5600