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Battery Isolator Switch


pearley

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8 hours ago, Rick-n-Jo said:

It's likely that this particular isolator is prone to failure because the Victron contains some big capacitors. The high inrush current causes severe arcing and burning of the contacts when switching on. If you want to keep an isolator in this circuit, a fix is to wire a 470 ohm resistor and push button switch in parallel with the isolator. Pressing the button for 15 seconds or so before switching on will gently pre-charge the capacitors and prevent the arc.

Is it not more likely to cause arcing when switching off?

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1 minute ago, Iain_S said:

Is it not more likely to cause arcing when switching off?

No. Assuming you switch it off after you have switched the Combi off at its own electronic switch and/or there is little current passing through it. When on, the capacitors are fully charged. If you switch off the isolator, any current demand continues to be fed by the capacitors and the voltage decays relatively slowly. If the Combi is off, it will probably take minutes or hours for the capacitors to discharge. But when you put the isolator on, you are connecting discharged capacitors to 12v by means of some very fat wires. You will recall that current flow in a capacitor is described by I = C.dV/dt and with dV/dt being theoretically infinite, so is the intital current. Theoretically of course, in fact it is limited only by the resistance of those fat wires connecting the battery to the Combi, internal resistance of the capacitors (which is a few milliohms). And of course the inductance of that wiring and of the capacitors, with the extremely large rate of rise of current the inductance is significant and of course the combination of inductance and capacitance is ideal to cause some ringing, which is what briefly maintains an arc across the burning contacts of the isolator.

 

From a practical point of view, I can report that disconnecting the fuse on our Combi is a non-event. But when that fuse is reconnected an hour or two later, the fat and vicious spark will make anyone unprepared, jump out of their skin!

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17 minutes ago, Iain_S said:

Is it not more likely to cause arcing when switching off?

Not if the inverter is switched off first (which you should)  so that no current is flowing.

 

Which is the short version of what Nick said!

Edited by Rick-n-Jo
Beaten to it
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On 26/05/2018 at 20:07, pearley said:

When new Joanie M had what looked like a cheap isolator switch for the Victron 3000/12/120 Combi and if happily sat there without being touched for 5 or 6 years passing charge current in one direction and running our 1600 watt washer in the other direction. One day it melted and I replaced it with one from a Chandlers that was rated as 300 amp continuous. This lasted about 4 years before it took melted. This coincided with a change of washer, same power consumption but washer, Zanussi, wouldn't work from inverter. Never the less I replaced the isolater again, this time with a Blue Sea one rated at 500 amp continuous, 1000amp  on overload. This is now 18 months old. 

 

Now we had a small bit of welding  done and turned all the isolators off. First time for 4 years. When turned back on all the LEDs on the inverter lit up as they should and the LCD reading on the remote display lit up. However, after a while it became apparent that the batteries were not being charged from mains. Panicking, I thought that the welding had damaged the charger despite the isolator being turned off. Checking voltages and fuses I found that it was charging, just wasn't getting to the batteries and traced the fault / high resistance to the isolator. Whacking the knob back and forward and the fault cleared. I removed the switch from its mounting and checked the connections behind. So assumption was that contacts were tarnished and now cleared. 

 

Came back from a day away and started the generator and all was well and the batteries charged from 88% to 96% before the gennie was switched off. Switched the gennie on again an hour ago to bring the batteries up to 100% and no charge again. Whack the switch back and forth again and now charging. 

 

So, any recommendations for an isolator that will last more that 4 years in continuous use, but rarely more than 150 amps. 

 

Mine are Blue sea.spot on.

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4 hours ago, pearley said:

How many liveaboards ever turn their isolators off? 

I have never turned any off or gas on any boat I have lived on. How many people throw their consumers panels off and isolate the gas in their house when they go out? I never did.

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38 minutes ago, Rick-n-Jo said:

Not if the inverter is switched off first (which you should)  so that no current is flowing.

 

Which is the short version of what Nick said!

Agreed. No current : no arc.

38 minutes ago, nicknorman said:

(snip)

From a practical point of view, I can report that disconnecting the fuse on our Combi is a non-event. But when that fuse is reconnected an hour or two later, the fat and vicious spark will make anyone unprepared, jump out of their skin!

I suspect the spark is caused by momentary connection/disconnection as the fuse is replaced, bearing in mind the breakdown voltage of air. A switch should not show the same effect, unless suffering from contact bounce.

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30 minutes ago, Iain_S said:

Agreed. No current : no arc.

I suspect the spark is caused by momentary connection/disconnection as the fuse is replaced, bearing in mind the breakdown voltage of air. A switch should not show the same effect, unless suffering from contact bounce.

Most switches have some sort of contact bounce, but anyway perhaps the mechanism is thus: as the switch is closed, initially the contact area is very small and the very large current over the very small area burns the metal, vapourising it or at least turning it to ash, this breaks the current so an arc forms to find a new path. Obviously all over in a flash (pun intended) but some contact burn whether it be a fuse connection or a “proper” switch.

 

My experience is that there is a powerful arc even though I deliberately attempt to bring the contact surfaces together cleanly and quickly.

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Galvanic corrosion of dissimilar metals? My isolator switches  are coming up to 30 years old, the house one copes with a big inverter without complaint.

Last year the engine cut out a few minutes after starting ( on the first spin over ) and switching that isolator off and on again restored power ( the fuel valve shuts without ignition power), not done it since.

 

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5 hours ago, nicknorman said:

Most switches have some sort of contact bounce, but anyway perhaps the mechanism is thus: as the switch is closed, initially the contact area is very small and the very large current over the very small area burns the metal, vapourising it or at least turning it to ash, this breaks the current so an arc forms to find a new path. Obviously all over in a flash (pun intended) but some contact burn whether it be a fuse connection or a “proper” switch.

 

My experience is that there is a powerful arc even though I deliberately attempt to bring the contact surfaces together cleanly and quickly.

Large current over small area maybe, but for very short time, and little energy expended. I have dealings with a DC switchboard, with old style knife switches, and when they are operated, there is sometimes a small spark as they are closed, and a big one every time when they are opened.(Always assuming that there isn't another downstream switch in the "off" position :) ) With some mains switches, the flash as it is operated can be seen through the plastic when they are operated in the dark, and every time the spark when opening is larger than the spark (if any) when closing. This does contrast with fuses being pulled, when there can be a bit of arcing when they are replaced. (Thinking about our home fuse box here, but the effect will be magnified by capacitance on the load side)

 

Anyway, we're getting a bit off topic, and we don't really know the failure mode of the OP's switch!

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4 hours ago, Iain_S said:

Large current over small area maybe, but for very short time, and little energy expended. I have dealings with a DC switchboard, with old style knife switches, and when they are operated, there is sometimes a small spark as they are closed, and a big one every time when they are opened.(Always assuming that there isn't another downstream switch in the "off" position :) ) With some mains switches, the flash as it is operated can be seen through the plastic when they are operated in the dark, and every time the spark when opening is larger than the spark (if any) when closing. This does contrast with fuses being pulled, when there can be a bit of arcing when they are replaced. (Thinking about our home fuse box here, but the effect will be magnified by capacitance on the load side)

 

Anyway, we're getting a bit off topic, and we don't really know the failure mode of the OP's switch!

Agree with you general points, but the case of the DC supply to a device with large capacitors on the input is different.

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As this is the only time the isolator has been switched off since I fitted it 2 years ago (I checked my diary, almost 2 years to the day) I'm going with the corrosion rather than the arcing theory. 

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58 minutes ago, pearley said:

As this is the only time the isolator has been switched off since I fitted it 2 years ago (I checked my diary, almost 2 years to the day) I'm going with the corrosion rather than the arcing theory. 

As would I. An email to BlueSea might generate a thoughtful response. 

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I have never taken a Blue Sea one apart but the crappy red key ones have no "wiping" action on the contacts when operated which most switches have. I suppose this is because they are not considered to be switches as such but disconnection devices.

Said red key ones frequently fail open circuit, the contact pressure seems very low.

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Stopped charging again today until isolator turned off and on a few times so hopefully on and took it out of the circuit and moved the mega fuse holder to where the switch was. 

 

Took if apart but looks brand new inside. 

20180528_154226.jpg

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21 minutes ago, pearley said:

Stopped charging again today until isolator turned off and on a few times so hopefully on and took it out of the circuit and moved the mega fuse holder to where the switch was. 

 

Took if apart but looks brand new inside. 

20180528_154226.jpg

Odd that. So do my wonderings about a lazy spring appear to have any validity?

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4 hours ago, pearley said:

Probably. 

Switch got hot with the high current flow. Spring got hot. Spring lost some... er... springiness. (I was going to write ‘temper’ but I wasn’t sure that was correct). 

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22 minutes ago, WotEver said:

Switch got hot with the high current flow. Spring got hot. Spring lost some... er... springiness. (I was going to write ‘temper’ but I wasn’t sure that was correct). 

Its a bit flawed though......to get hot, it would need to have a resistance sufficiently high to allow such a PD drop to cause it to dissipate the energy, and before then where would the PD drop have developed....with that cold, springy spring doing its job and holding the contacts closed.  I say the contacts would need to be corroded, or have some kind of deposit/build up on them to make this happen. This would be the initiating step. Then  subsequent steps might be that the thing got hot, then the spring loses its springiness, then it has more of a resistance etc in a vicious circle. 

 

But how did it get hot in the first place and the spring lose its springiness? And what is the design factor on the spring anyway?

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15 minutes ago, Paul C said:

Its a bit flawed though......to get hot, it would need to have a resistance sufficiently high to allow such a PD drop to cause it to dissipate the energy, and before then where would the PD drop have developed....with that cold, springy spring doing its job and holding the contacts closed.  I say the contacts would need to be corroded, or have some kind of deposit/build up on them to make this happen. This would be the initiating step. Then  subsequent steps might be that the thing got hot, then the spring loses its springiness, then it has more of a resistance etc in a vicious circle. 

 

But how did it get hot in the first place and the spring lose its springiness? And what is the design factor on the spring anyway?

Heck, I dunno ;)

 

Maybe it got hot because... umm... yeah, it’s a flawed concept...

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