Re: Understanding breakers


Mark McGovern
 
Edited

Pat,

I will try to respond directly to your post with your parts in BOLD:

I have posted that I have a couple of breakers that pop. The charger is what most concerns me,it ran for ten minutes this morning then popped off. I switched it over to the heater breaker also 25A and it popped after a few minutes,but now is fine for the last 20 minutes. Both breakers are 20 years old and could be weak. If I had a problem with the charger , would it not be constant and pop the breakers immediately.? 

If you had a short in the circuit supplying power to the charger it would trip the breaker almost immediately.  However, if the Diruptor 25 amp circuit breaker is a thermal breaker (and I believe that they are) and the amount of current through the breaker was only a small amount over the rating (say 30 amps) than it would take time for the heat to build up in the bimetallic strip inside the breaker and cause it to trip.  Other and more likely things besides higher than rated current flow can cause overheating and delayed tripping of a thermal breaker, mainly corrosion or a loose connection in the circuit

What does the breaker exactly do? 

As Brett's post explained, the breaker is designed to protect the wires in the circuit in the event that on overcurrent situation arises.  It essentially prevents electrical fires from starting.  A good explanation of typical circuit breakers can be found here:  https://electronics.howstuffworks.com/circuit-breaker.htm

It is only 25A and the charger is putting out 75A so the current to the battery does not flow thru it.

Correct.  The 25 Amp breaker is in the circuit that is providing 220 volts of AC power to the Charger.  It is NOT in the circuit that goes FROM the Charger TO the batteries.

Does it close a relay between the genset and the charger? 

No.  It is directly wired to the battery charger.  There is nothing in between it and the battery charger except for wires.  

The only "connection" to the Genset happens BEFORE the main AC power panel.  The main AC power Panel is powered by EITHER the Genset OR Shore Power.  The flow of 220 volt AC power goes something like this:  Genset OR Shore Power -->  40 AMP RCD Breaker  -->  Main AC Power Panel  --> 25 Amp Circuit Breaker for 100 Amp Battery Charger  -->  100 Amp Battery Charger

But why would that be 25A?

The 25 Amp breaker on the AC Panel is operating at a nominal 220 volts.  The circuit provides 220 volts of AC power to the Charger.  The circuit breaker allows UP TO 25 Amps to flow through the circuit.  It might be easier to think of Power in Watts especially when dealing with both AC and DC circuits.  Watts = Volts x Amps.  So the AC circuit powering the battery charger can handle 220 Volts x 25 Amps = 5500 Watts.
 
The Battery Charger converts the 220 volts AC to nominal 24 volts DC.  It outputs a nominal 24 volts UP TO 100 Amps.  So the maximum wattage is 24 Volts x 100 Amps = 2400 Watts.  This is significantly lower than the maximum of 5500 watts of power supplied which makes sense. 

Note that the circuit providing 24v power to the batteries is a completely different circuit than the AC circuit that provide power to the Battery Charger.

I am thinking that I could get another 25A 220v breaker from a hardware store and substitute it and if it doesn’t pop ,then it confirms I have a bad breaker. I will contact Maude,but we want to leave Fla. for Bahamas before we would get it, if she even has one. The charger has now ran for 45 minutes no problem. Please explain the function of the breaker, I am uncertain about what it is powering..

The fact that two different breakers trip in a similar manner makes it unlikely that you have a faulty circuit breaker.  This is not 100% certain given that both breakers are 20 years old, but the chances of both breakers failing in the same manner at the same time is pretty small.  Given that it took some time for the breaker to open rather than tripping almost immediately, means it is NOT a massive overcurrent like a short circuit.  The most likely cause of a breaker tripping like this in a marine environment is either corrosion on the wire terminals or a loose connection at one or more of the terminals.  Corrosion or a loose connection will both cause heat to build up in the connection and eventually in the thermal circuit breaker itself causing it to trip.

You may have inadvertently fixed the original cause of the problem when you swapped out the breaker either by fixing the loose connection or mechanically removing some corrosion on the wires.  However, I would also take a look at the connections of the 220v wire inside the battery charger just to be sure.

I hope that this helps.  "Real life" electrical engineers or electricians here feel free to correct any mistakes in my understanding or explanation.

Mark McGovern
SM #440 Cara
Brunswick, GA USA

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