Periodic inspection - money well spent

[JohnW2]

I'll bet that an AC rated MCB switching DC would do the contacts in.

Not necessarily. When breaking the supply on a fault, there's a high power supply so when the contacts open, there's the voltage and energy to initiate a spark. Basically, you've got a supply capable of putting 240V and hundreds of amps into the spark. ...

I don't think so. The energy generated by an arc is dependent only on the current and the resistance of the arc (which is extremely low). IIRC typical arc voltages (voltage across arc) are of the order of 10-20V/cm, so when the contacts have only opened by, say, 1mm, that will be 1-2V, regardless of of what the 'supply voltage' is. In fact, the contacts (and what's happening between them) haven't got a clue what the 'supply voltage' is whilst the arc is present (since the arc effectively keeps the contacts 'closed'). Only when the arc is broken (and the contacts become truly 'open') will the voltage across the (now open) contacts rise to the supply voltage - but then, of course, it no longer matters. AFAICS, a given current will therefore result in the same arc energy (when contacts attempt to open), regardless of what 'supply voltage' is responsible for that current.

Kind Regards, John

 

[Dave54]

Makes me think of all that electrical theory learned in those long afternoon lectures.
And mostly forgotten now from being unused!

DC contactors for heavy loads have (relatively to gear designed for AC) large "break" gaps to ensure there is sufficient gap to break the arc, as well as heavy arc shutes and arc blowout coils to minimise arcing.
The old (And I'm talking about pre WWII) porcelain and metal light switches were rated for AC or DC, and had a "Knife switch" mechanism inside which when operated again gave a large air gap.
Not a problem with AC of course because the voltage cycles through zero and so helps to break any arc present.

 

[SimonH2]

I don't think so. The energy generated by an arc is dependent only on the current and the resistance of the arc (which is extremely low). IIRC typical arc voltages (voltage across arc) are of the order of 10-20V/cm, so when the contacts have only opened by, say, 1mm, that will be 1-2V, regardless of of what the 'supply voltage' is.

Yes, but how long will that arc persist when there's only a couple of volts to drive it ? As pointed out, the problem with DC vs AC is that the arc is persistent rather than extinguishing 100 times a second - so will grow longer and persist longer than an equivalent arc on a similar AC voltage.
So in that respect, yes an arc will form - as it will under normal operation on AC. But it will extinguish a lot faster, and hence do less to the contacts, than it would with 240V AC driving it.

And apart form that, with my though about variac + welding transformer setup, there's the option of AC as long as I pick the right connection points.

I do recall when I was an apprentice, for a while we had a 'kin big transformer in the workshop when they needed to test some large breakers before they went on the boat. From memory it must have been about twice the size of those 1000l IBC containers and was capable of putting (at least) a few hundred amps through the contacts. IIRC there had been some problems with contact resistance, and there's been some contact cleaning carried out and they needed to test the results. The breakers weren't much smaller than an IBC either.

 

[Dave54]

As far as I know (after John reminded me anyway! ) the low voltage will maintain the arc "indefinitely". That's the problem. It requires the voltage to drop below that dependent on the air gap.
Another point that's just occurred to me; DC from a battery has to be the worst sort to break because it's a steady voltage? There's no ripple of any sort.
Slightly OT. An interesting thing told me long ago by a rep from one if the big switchgear makers was that in their 500 Amp contact breakers the actual contacts were in semi liquid state when conducting, hence they were self cleaning and no maintenance was required. I opened one of the units after about 12 months, and sure enough there were sort of "ripples" on the surface of the contacts, but minimal pitting or burning.
They were rated AC only.
Checking and cleaning the contacts on both AC and DC contactors was a regular job with the old type gear.

 

[SimonH2]

That's the problem. It requires the voltage to drop below that dependent on the air gap.

Well exactly.
When working on mains, you've got 240V AC, so around 340V peak to maintain the arc. And with a supply capable (at least for a short while) of supplying a few hundred amps, so say you have 400A, that's 100kW sustainable for (in this context) a significant time. Given the typical prospective fault currents available, this may well be an understatement of the energy available (note that we often have the option of 6kA or 10kA capable breakers !) Looking at past PIRs, I see one of my properties has a prospective fault current of 3.1kA - so in theory up to 744kW of energy going into an arc of negligible resistance (for a very short time)
With a low voltage, you've (say) 5V - even if you've arranged for 1000A to be available, that's only 5kW maximum arc energy. 10kW if your test rig is 10V and so on. Just the impedance of the test gear (transformers etc) is going to significantly restrict the arc energy.

 

[Dave54]

No real problem with AC at mains voltage though ia there? Very small air gaps between contacts are sufficient, because as already said the supply, and hence the voltage across the arc regularly cycle through zero.
If you put even fairly low DC voltage onto an AC rated MCB from a car battery then a very high current of several hundred Amps will flow. Assuming a 5mm gap at 10V /cm that's (say) 2.5 kW at a very high temperature in a small enclosure.
The mcb isn't going to last long.

 

[JohnW2]

Yes, but how long will that arc persist when there's only a couple of volts to drive it ?

Particularly with DC, the arc will perists indefinitely, regardless of what voltage is 'driving it'. As I said, so long as the arcs persist, neither the arc nor the contacts have any clue as to whether the supply responsible for the current is 1V, 100V or 10,000V.

Kind Regards,

John

 

[JohnW2]

When working on mains, you've got 240V AC, so around 340V peak to maintain the arc. And with a supply capable (at least for a short while) of supplying a few hundred amps, so say you have 400A, that's 100kW sustainable for (in this context) a significant time.

Again, that's irrelevant to an arc. Maybe there is 100kW being dissipated somewhere, but the resistance of the arc is so low that only a tiny proportion of the power will be dissipated there. The great majority of the resistance of the current loop (hence most of that 240V/340V, and the great majority of the dissipated power) will be in the wiring of the installation and supply network - and only a tiny voltage will appear across the arc.

As I said, arc voltages (the voltage across the arc) are, IIRC, typically 10-20V per cm (and I think that hold roughly for currents up to about 50 kA) - so, as I said, 1-2V when the contacts are 1mm apart. ... and, again, that is totally regardless of the voltage of the source responsible for the high current.

Kind Regards, John

 

[JohnW2]

No real problem with AC at mains voltage though ia there? Very small air gaps between contacts are sufficient, because as already said the supply, and hence the voltage across the arc regularly cycle through zero.

Indeed, but remember that this whole discussion started because of proposals to test MCBs with DC, whereupon someone said (I believe probably correctly), that breaking a DC current high enough to magnetically trip an MCB would probably destroy (or, at least, damage) the contacts. Hence, such a test might confirm that an MCB was functioning correctly but, as a result of the testing, it would probably have to be thrown out

Kind Regards, John

 

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