diy solar

diy solar

Testing Magnetic-Hydraulic circuit breakers


I See Electromagnetic Fields!
Mar 28, 2020
I've built myself a circuit breaker tester.
I assembled three transformers into a 3-phase wye auto-transformer, and by means of a breaker and cord/plug it can be repurposed into a current multiplier. A breaker connects X1 to H2 and H3, providing 4.7x current boost on X2 paralleled with X3. Electric heaters set a regulated current, and the higher multiplied current is wired through "DUT" breaker to neutral. A Variac feeding one of the heaters allows fine adjustment.

Variac breaker tester IMG_2713.jpg

I've bought the following two Midnight breakers, and peeling back their sticker reveals CBI's part number:

MNEAC60-2P = CBI QZ-2(13)-DM-2-60A
MNEAC60QZD-2P = CBI QZ-2(13)-D-2-60A

MNEAC60-2P IMG_2723.jpg

Although one is described as 120/240V on (a different) Midnight's page, both have 277/480 printed on them and spec'd at CBI's site.

The interesting data is here, including delay curves:

"-2" on each breaker indicates curve #2.
What the curve shows is guaranteed never to trip below 105% of rating, to trip after 10 minutes at 130% of rating.
1500% of rating, fast trip takes at least 0.003 seconds, at most 0.020 seconds.
Between those extremes, hydraulic delay means 300% of rating takes between 2 and 12 seconds to trip.

This article from another brand explains the hydraulic delay.
A spring loaded core moves though silicone oil, and when it reaches the end, increased magnetic field actuates trip mechanism.

My interest in these is to provide reliable operation with Sunny Island. Max current is 56A, max breaker for input is 70A (for output 56A??).
I've had difficulty with others causing current imbalance in parallel circuits, and nuisance tripping on a high-PV production day.

With these Midnight breakers, just installed, I'm seeing 23A on one, 24A on the parallel path, so pretty well matched.

Using the tester, I observed 117A being carried for a number of seconds, then tripping.
70A (125% of rating), it tripped after a while. 65A (just above 105% or 63A no-trip spec) it carried without tripping. My 56A max application should be good.
Various tests, I thought it tripped faster the next time, and was getting warm. So I was suspicious it might actually be thermal-magnetic. But it is operating to spec, and there is likely a delay for core to move back under spring pressure (no check valve.)

117A IMG_2718.jpg