ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Well, actually it was a couple of days ago, but it was news to me.
I am working on a 812kW PV system that will interconnect load side at two points in a large MDP through a 350A and a 800A breaker. When the breakers came in I found that they are dual trip - thermal and magnetic - and the magnetic section is adjustable. I had never seen that before.
The thermal section uses the same bimetallic strip that we all know about, which can take a long time to trip, especially if it sees an overload only a little above its rating. The magnetic trip point is adjustable on the 800A breaker from 4X to 8X of the breaker rating and the 350A is adjustable from 5X to 10X, and it trips nearly instantaneously. What does this mean and why is it there?
In a word, it's there for inrush. If the breaker is feeding a large transformer, the inrush when the breaker is closed is significant and can momentarily be several times the rating of the breaker, though it will be less than a short circuit would draw. The magnetic trip point is set to enable the breaker to distinguish between inrush and a short circuit fault. Whatever equipment you have the breaker connected to, you want this setting to be as low as possible without causing nuisance trips, because if it is set too high, in the event of a short on the line it can cause another breaker farther upstream to trip instead and thereby shut down more of the system than is necessary to clear the fault.
So, what about inverters? How much inrush do they draw when they are connected to the AC bus? According to SMA tech support, very little, at least in the case of transformerless inverters - something in the milliamp range per inverter - so even in the system I am building with 30+ SMA Tripower inverters in parallel it is nearly nothing. The magnetic trip points on the breakers should therefore be set at their lowest value because any current draw in excess of the breaker rating can only be due to a fault.
Anyway, this was a new wrinkle for me, so I thought I would share it.
I am working on a 812kW PV system that will interconnect load side at two points in a large MDP through a 350A and a 800A breaker. When the breakers came in I found that they are dual trip - thermal and magnetic - and the magnetic section is adjustable. I had never seen that before.
The thermal section uses the same bimetallic strip that we all know about, which can take a long time to trip, especially if it sees an overload only a little above its rating. The magnetic trip point is adjustable on the 800A breaker from 4X to 8X of the breaker rating and the 350A is adjustable from 5X to 10X, and it trips nearly instantaneously. What does this mean and why is it there?
In a word, it's there for inrush. If the breaker is feeding a large transformer, the inrush when the breaker is closed is significant and can momentarily be several times the rating of the breaker, though it will be less than a short circuit would draw. The magnetic trip point is set to enable the breaker to distinguish between inrush and a short circuit fault. Whatever equipment you have the breaker connected to, you want this setting to be as low as possible without causing nuisance trips, because if it is set too high, in the event of a short on the line it can cause another breaker farther upstream to trip instead and thereby shut down more of the system than is necessary to clear the fault.
So, what about inverters? How much inrush do they draw when they are connected to the AC bus? According to SMA tech support, very little, at least in the case of transformerless inverters - something in the milliamp range per inverter - so even in the system I am building with 30+ SMA Tripower inverters in parallel it is nearly nothing. The magnetic trip points on the breakers should therefore be set at their lowest value because any current draw in excess of the breaker rating can only be due to a fault.
Anyway, this was a new wrinkle for me, so I thought I would share it.
