I keep blowing 400 amp t-fuses when charging at 240 amps

JedTheFed · Jan 21, 2022

Hello everyone,

Scenario

10 KW onan generator

cerbo gx set to accept 50 amps

800 amps renogy batteries

2 multiplus 3000 wires in split phase.

400 amp t fuse

I set both inverters to charge at 120 amps the 400 amp t-fuse blows. If I set them both to 100 amps charging it seems ok. Why is this happening?

I thought it was because my solar charger was on at the same time when the first fuse blew, but it is still happening. I was bringing the power up from 62% charge and it got as far as 71% before the last fuse blew. This time it even melted the fuse holder.

Can I not use the full 240 amps charging? It seems I could do this before but can not do it recently without the fuse blowing.

Why is 240 amps charging blowing a 400 amp fuse?

Gary

sunshine_eggo · Jan 21, 2022

Most common causes of premature fuse blow:

Bad connection at fuse that's generating heat.
Low quality cheap fuse that doesn't actually meet spec.

BentleyJ · Jan 21, 2022

Since the fuse holder melted at only 71% current its a heat issue caused by a connection with too much resistance.

JedTheFed · Jan 21, 2022

Than

sunshine_eggo said:
Most common causes of premature fuse blow:

Bad connection at fuse that's generating heat.
Low quality cheap fuse that doesn't actually meet spec.

Thank you for your response. The t-fuses seem to be good quality and were tightened to inch pounds spec (i can’t recall what that was at the moment). I will definitely check that again.

John Frum · Jan 21, 2022

What is the dc system voltage?

John Frum · Jan 21, 2022

Also what is the system topology?
Where is the fuse in relation to the batteries and the loads/charge sources?

JedTheFed · Jan 21, 2022

BentleyJ said:
Since the fuse holder melted at only 71% current its a heat issue caused by a connection with too much resistance.

The charge amps were a steady 230+ and the battery state of charge had climbed to 71% when the last one blew. I was definitely be checking the torque on the fuse next.

In another forum it was suggested it was just getting hot from being in a hot bay with the inverters, batteries and solar controller all together. Maybe my temp sense is not setup correctly.

Gary

JedTheFed · Jan 21, 2022

smoothJoey said:
What is the dc system voltage?

12 v system

JedTheFed · Jan 21, 2022

smoothJoey said:
Also what is the system topology?
Where is the fuse in relation to the batteries and the loads/charge sources?

Fuse is inline between inverters and batteries.

John Frum · Jan 21, 2022

JedTheFed said:
Fuse is inline between inverters and batteries.

Could this be really fast blow fuse that is tripping on inverter in-rush?

time2roll · Jan 21, 2022

Inch pounds on a 400? I would lean on it a bit harder. Polish the surface clean also.
Get your DC clamp ammeter on the fuse feeder to verify current.

JedTheFed · Jan 21, 2022

smoothJoey said:
Could this be really fast blow fuse that is tripping on inverter in-rush?

It is a 400 amp tfuse slow blow fuse

John Frum · Jan 21, 2022

JedTheFed said:
It is a 400 amp tfuse slow blow fuse

Can you post a product link to it?
I've not seen a slow blow class t fuse and I went looking a few monthes back.
The demand for class-t fuses is high right now, as you have noticed many places are out of stock.
I wonder if some reject fuses got into the distribution channel so somebody could make a quick buck.
If the fuses are not blowing when the inverter/chargers power on then its not capacitor in-rush.
Is that what is happening?

time2roll · Jan 21, 2022

Blue Sea chart says the T should pass about 2x rating for close to a minute.
Seems slow to me. I prefer to keep these a bit tight without worry of inrush or surge rating.

HRTKD · Jan 21, 2022

What part of the fuse holder melted? Can you post a picture?

JedTheFed · Jan 21, 2022

time2roll said:
Inch pounds on a 400? I would lean on it a bit harder. Polish the surface clean also.
Get your DC clamp ammeter on the fuse feeder to verify current.

Thank you for thart, polished and tightened again

JedTheFed · Jan 21, 2022

smoothJoey said:
Can you post a product link to it?
I've not seen a slow blow class t fuse and I went looking a few monthes back.
The demand for class-t fuses is high right now, as you have noticed many places are out of stock.
I wonder if some reject fuses got into the distribution channel so somebody could make a quick buck.
If the fuses are not blowing when the inverter/chargers power on then its not capacitor in-rush.
Is that what is happening?

Go power 400 amp fuse. I blew 3 of them

Go Power F-400 - Inverter Supply

Shop for Go Power F-400 at Inverter Supply for best prices online. Go Power F-400, Go Power, F-400, 400 Amp Slow Blow Fuse Class T, Go Power Replacement Fuses

www.invertersupply.com

JedTheFed · Jan 21, 2022

time2roll said:
Blue Sea chart says the T should pass about 2x rating for close to a minute.
Seems slow to me. I prefer to keep these a bit tight without worry of inrush or surge rating.

It does seem long and it was HOT! I am considering switching to a 400 amp breaker and tweaking the settings to slow down the surge.

JedTheFed · Jan 21, 2022

HRTKD said:
What part of the fuse holder melted? Can you post a picture?

Where it melted was where the bolts go through the plastic. I tried removing the fuse while it was still smoking hot and this screwed up the plastic a bit. Its all back together and looks normal again until I replace it.

time2roll · Jan 21, 2022

You can put a lot of torque on a 3/8 bolt. Tighten it up and see how it goes.

I keep blowing 400 amp t-fuses when charging at 240 amps

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