Surge supression for phase to neutral shorts

[Thermoelectric]

Hey,

Run into an interesting problem at work. I'm an electrician and we have these cranes which move pretty fast, powered by brushes running on bus bar. Multiple circuits in the bus bar, a 240v safety circuit energising some relays, a 415v control circuit and 415v 3 phase power for the drive motors.

As these bus bars wear, we get copper strands that bridge out the brushes. Usually this just trips the breaker and clears the fault, however a short between phase and neutral blows up the 230v relay coils. In my mind the relatively high impedance of the bus bar turns it into a voltage divider during the fault event, likely pulling neutral up to 100+VAC until the circuit breakers trip.

I've been looking into it, essentially I need to keep neutral close to earth potential to stop this voltage rise on the relays, or put surge suppression across the relay coils. These Finder relays already have the small MOV based surge suppressor but it is mainly designed to reduce transients from the coil being switched, and these blow up as well. I believe these also cause a cascading failure which takes out the second relay in some cases.

My intent is to install something to stop the relays blowing up and requiring semi-frequent replacement. My first thought was a single phase surge arrestor e.g. a HAGER SPB140D across each relay. Clamps at >285V and limits the voltage the relay coil experiences. Alternatively a low voltage transient barrier such as a Critec UTB-30 between neutral and earth to limit rise to no more than 30v.

Upon further investigation, surge arrestors such as these are rated for very short impulses (8/20µs), not the 10ms it could potentially take the control circuit breakers to trip. I am starting to think that these surge arrestors would not last, taking the long and low current impulses.

Anyone have any suggestions for an off the shelf part that would potentially do what I'm looking for? I have yet to measure the fault current but anticipate it to be rather low, in the 200A or less range. Ideally whatever it is needs to act fast like a surge arrestor, but clamp a fault for up to 10ms, and last hundreds of cycles ideally.

Rough drawing attached. Short which causes the relay damage occurs between L1A & N, or L1B & N.

Cheers!

[jonpaul]

Bonjour cher Monsieu, à Very fine post and problem

The energy storage in industrial applications can be far beyond the capability of the small MOV, and other surpression, many joules of stored energy. Formule is 1/2 L I sq with L as coil or motor inductance in Hy and I as peak current in ampères.


A current probe and digital scope may capture the transient.  Alternative rent a line transients recorder that
uses safety rated V and I probes on the device.


The problem is similar to automotive industry alternator load dump transient, 10..100 MS.
Contact some industrial transient surpression firms,

Finally, there is a great electrician forum for professionals, run by Mike Holt. Consider to post there.

https://forums.mikeholt.com/forums/general-electrical-forum.44/

He is a top expert.

Bon chance


Jon

[Thermoelectric]

Thank you very much for your reply.

Unfortunately measuring this fault will be difficult as it happens very sporadically, and I'm not sure my boss will like me intentionally causing it for testing purposes. However your comparison to the alternator load dump transient is wonderful! Some research into that shows TVS diodes seem like they would be a perfect fit for the application, limiting voltage rise between neutral and earth. These seem to come in quite substantial power dissipation ratings as well.

I'll have to try and measure the fault current at the control cabinet to see what the worst case current will be. Hardest part of this all may be finding a suitable off the shelf TVS assembly, would be very nice to find something DIN mount for this application.

Cheers!

[NiHaoMike]

Add a series resistor to limit the current that the MOV needs to clamp?

[mag_therm]

That circuit as shown might not meet the Australian wiring regulations with the break of guage.
for the relay wiring, and upstream fuses at the source of the crane rail.

Special fuses called "semiconductor fuses" or "high speed fuses" could be selected for the relays.

Like these, see the 5 Amp one.
https://www.farnell.com/datasheets/2151267.pdf
A different case or brand  might be needed to fit in the standard holders at the plant

Calculations are required (page 235 for the curves) so that the MOVs or TVS can survive the impulse while the fuse clears.
It might be necessary to uprate the MOVS to give a margin of safety over the fuse let-through I^t.

[jonpaul]

Rebonjour Cher Thermoelectric

suggest that you check out the "whole house" service entrance mains surge  protectors, 240V service, very large die safety rated surge/ lightning  protection, perhaps in DIN rail?

We have in USA Cutler Hamer, Square D, Eaton,  in Australia.  ABB, Schnieder?

Perhaps these mains surge  protectors can be adapted to your site

Interested in the solution,


À Bientôt

Jon

[ajb]

Tricky problem! Does the 240VAC control circuit use the same neutral rail/brush as the 415VAC power circuits?  Having a separate neutral contact for it would give you a few options to isolate the fault from hitting the relays versus trying to protect them against it.  Could you convert the control circuit to 415V? That would at least reduce the magnitude of the overvoltage event from the relays' perspective.

A semiconductor fuse plus a beefy industrial surge protector is probably the best bet for protecting the relays, although having to replace a fuse still means having to open a cabinet which might reduce the benefit versus a solution that would let you just reset the breaker and carry on. 

[TheMG]

More frequent maintenance/cleaning of the bus bars and brushes to prevent such shorts from happening in the first place?

Also some small fast-acting fuses prior to the relay coils. When the MOV clamps down on the voltage, the fuse blows before either the relay or MOV get damaged. Right now the MOV are likely burning up and not doing the job since they are trying to dissipate far more power than designed to. Adequate fusing could reduce that to just fractions of a second and hopefully spare both the relay and MOV, put a new fuse in and you're up and running again.

[T3sl4co1l]

Assuming the description is accurate to the events, it seems most likely a surge occurs when the fault clears, whether because the material itself blew out, or the breaker did first.  In either case, some transient voltage could result, and it would be propelled by up to the fault current rating of the circuit -- which at 415V could be quite high indeed (100kA?).  Presumably it's a lot less than that (either it's not actually giving a full-blown arc flash discharge, or you're a very understated kind of person..!).  More likely the arc remains conductive down to some cutoff current (10s A?), and only then the transient goes off?

If the phenomena is this way, then the inductance in play is mainly the wiring between pad transformer and switching location, and that will likely give a fairly short duration transient,which a MOV would be capable of handling.

It might even be enough to add an SPD inline, near the bus bar connections say.  That would avoid modifying much circuitry, and utilizes approved equipment.  (Install according to recommendations and code, of course.  Maybe this isn't an appropriate location; maybe it'll be less effective if placed elsewhere, no idea.)

As for the relays, if they're dying to overvoltage, the easiest way to deal with that would indeed be a MOV, but considerable current could flow through that path.  A series resistor (of wirewound and pulse-rated type) could drop negligible voltage under normal conditions (e.g. if the coils are 100VA, that's about half an ampere, and 5% drop is 12V, or 24 ohms (say 20 or 22 for standard/common values), and 12V * 0.5A = 6W so a 10W+ resistor would suffice, and then even a small MOV will clamp transient voltages at the coil (dumping the difference across the resistor, hence the pulse rating).  MOV size would then be determined by duration of surge/fault -- absorbed energy.  If it's only arcing/inductive kick as speculated, probably even the smallest MOV would do (7mm 275VAC something or other?).  If it's actually more of a cross-wiring fault (415V appears across the coil), there could be sustained overvoltage (10s, 100s ms?) which might want a larger MOV (how much larger, isn't so obvious though).

It seems like, if it's sustained overvoltage, it would have to be seconds, minutes even; and that should be evident on autopsy (windings are burnt/melted).  Hrm, well, careful; if the winding breaks down due to overvoltage, you'd have to go digging for the initial spot and determine (after the fact) if it was dielectric breakdown or overheating.  Well, overheating should be the whole thing, whereas breakdown would spare (short circuit around) part of the coil.  That might be easy enough to see..?

Also some small fast-acting fuses prior to the relay coils. When the MOV clamps down on the voltage, the fuse blows before either the relay or MOV get damaged. Right now the MOV are likely burning up and not doing the job since they are trying to dissipate far more power than designed to. Adequate fusing could reduce that to just fractions of a second and hopefully spare both the relay and MOV, put a new fuse in and you're up and running again.

A semiconductor fuse plus a beefy industrial surge protector is probably the best bet for protecting the relays, although having to replace a fuse still means having to open a cabinet which might reduce the benefit versus a solution that would let you just reset the breaker and carry on. 

Indeed, semi fuse would be the fastest option; that can clear a fault in less than a line cycle (typically ~1ms at fault currents!).  With a typical fuse, one waits a cycle or two for current to pass through zero, and then the fuse opens.

This has the advantage that it's not vulnerable to continuous breakdown, as my resistor+MOV suggestion is; the disadvantage is, if this is a regular event, fusing may be a nuisance.

(Mind, the resistors should be fused too, these aren't exclusive options!  Thermal fuses could even be used to protect against MOV failure -- typically the MOV fails short, drawing fault current, in this case limited by the resistor but rapidly overheating it.  Thermal fuse on MOV, resistor, or both I suppose, would be something to consider)

Tim

[coppercone2]

lol what this made me think of is some kind of MOV device that is inside a light bulb base that you screw into the ceiling with one of those light bulb changing poles

[Thermoelectric]

That circuit as shown might not meet the Australian wiring regulations with the break of guage.
for the relay wiring, and upstream fuses at the source of the crane rail.

Special fuses called "semiconductor fuses" or "high speed fuses" could be selected for the relays.

That circuit drawing was slapped together just to demonstrate the problem but is essentially as per the actual installation, sharing the neutral between breakers like they do is a little odd however the bus bar is apparently rated at 63a, so I don't think the gauge downsizes anywhere below what's specced for the breakers. Semi fuses would likely work however if we have to replace fuses instead of relays, it still means replacing parts which is what I'm trying to avoid, essentially reset the breaker after a fault and all is sweet is the aim.

suggest that you check out the "whole house" service entrance mains surge  protectors, 240V service, very large die safety rated surge/ lightning  protection, perhaps in DIN rail?

This was my first thought, however the long duration of the potential fault event (up to 10ms while the circuit breaker clears) seems very damaging to MOV's. Littelfuse has a training document with the attached graph showing long duration faults reduce lifespan of MOV's substantially (graph attached at end of post).

Tricky problem! Does the 240VAC control circuit use the same neutral rail/brush as the 415VAC power circuits?  Having a separate neutral contact for it would give you a few options to isolate the fault from hitting the relays versus trying to protect them against it.  Could you convert the control circuit to 415V? That would at least reduce the magnitude of the overvoltage event from the relays' perspective.

A semiconductor fuse plus a beefy industrial surge protector is probably the best bet for protecting the relays, although having to replace a fuse still means having to open a cabinet which might reduce the benefit versus a solution that would let you just reset the breaker and carry on. 

240V two phase safety circuit (I think this is the main problem here, having these safeties on one phase would eliminate overvoltage issues caused by a short, ignoring transients) running the two relays and the 240v control circuit share a neutral. The main drive 3 phase 415V runs without a neutral, only really powers the VFD's on the crane which don't need the neutral. No bus bar for it.

You're on the money with my intentions, trying to avoid having to go and replace parts at all. I think I might investigate getting those two relays on the same phase, I don't see any particular reason they would need to be on two phases like that. In turn this would get rid of L2 from that section of the bus bar, which seems to be the root of the problem.

More frequent maintenance/cleaning of the bus bars and brushes to prevent such shorts from happening in the first place?

Also some small fast-acting fuses prior to the relay coils. When the MOV clamps down on the voltage, the fuse blows before either the relay or MOV get damaged. Right now the MOV are likely burning up and not doing the job since they are trying to dissipate far more power than designed to. Adequate fusing could reduce that to just fractions of a second and hopefully spare both the relay and MOV, put a new fuse in and you're up and running again.

We clean the bar every three months, and usually again when we get faults. However as I've recently experienced, you can never get all the little copper whiskers. Still got a small flash from one crane after we cleaned the bar and re-energised it, as it found a bit of copper we didn't manage to clean out. The main build up is carbon from the brushes. Fuses are definitely a consideration, will have a bit more of a think about it.

Assuming the description is accurate to the events, it seems most likely a surge occurs when the fault clears, whether because the material itself blew out, or the breaker did first.  In either case, some transient voltage could result, and it would be propelled by up to the fault current rating of the circuit -- which at 415V could be quite high indeed (100kA?).  Presumably it's a lot less than that (either it's not actually giving a full-blown arc flash discharge, or you're a very understated kind of person..!).  More likely the arc remains conductive down to some cutoff current (10s A?), and only then the transient goes off?

If the phenomena is this way, then the inductance in play is mainly the wiring between pad transformer and switching location, and that will likely give a fairly short duration transient,which a MOV would be capable of handling.

It might even be enough to add an SPD inline, near the bus bar connections say.  That would avoid modifying much circuitry, and utilizes approved equipment.  (Install according to recommendations and code, of course.  Maybe this isn't an appropriate location; maybe it'll be less effective if placed elsewhere, no idea.)

It seems like, if it's sustained overvoltage, it would have to be seconds, minutes even; and that should be evident on autopsy (windings are burnt/melted).  Hrm, well, careful; if the winding breaks down due to overvoltage, you'd have to go digging for the initial spot and determine (after the fact) if it was dielectric breakdown or overheating.  Well, overheating should be the whole thing, whereas breakdown would spare (short circuit around) part of the coil.  That might be easy enough to see..?

You've raised a point with regards to transients I had glossed over previously. This fault always blows one relay, sometimes two. The first relay that blows is 100% overvoltage, 415v on a 240v coil style. Wire from the pin on the bottom of the relay to the coil fuses and vaporises, leaving the nice pretty shiny copper plating inside the relay. The MOV on this relay also turns into a charred mess. So, when the short on the bus bar occurs this overvolts the relay and MOV, killing them.

However, the second relay didn't make much sense to me at first as the short causing the other relay to blow up would reduce voltage on the second relay. Your point regarding transients makes much more sense here, once that short clears, and the first relay/MOV die, you get the transient which would be killing the second relay. The single fault I've witnessed so far only nailed the first relay so the autopsy of the second yielded no results.

With regards to fault current, I'm going off testing I've performed previously on similarly fed 3 phase installations (63a, 30+m of cable prior to the cabinet, few hundred meters from site transformers), where prospective fault current is around 1kA max. After you get through the breakers and smaller cable going to the bus bar I'd imagine the fault current is quite a bit lower still. Will measure next time I'm in.

As for the parallel surge arrestors, this is a possibility I have considered however the graph I found from Littelfuse (attached below) suggests a long duration(10ms), "low" current (200a) fault induced by the short, not the transient, will shorten the MOV's life substantially. The tiny MOV's on the relays may be enough to take the transients, haven't got specs on these MOV's yet.

[jonpaul]

however the long duration of the potential fault event (up to 10ms while the circuit breaker clears) seems very damaging to MOV's


The "whole house" surge devices have a lot more then just a consumer MOV.

They are designed for repeated surges eg utility cutout and reclose, thus dump of utility transformer energy.

The surge protectors are also made in very large industrial versions.

See "Utility surge (or) Lightning Arrestor"

Photo is 12 kV and up....
Jon

[T3sl4co1l]

Easy enough to model:



The box is my energy meter model, it just integrates V*I at its terminals.  Power is postprocessed as EMOV1/(time+1u), simple enough.




The first half, 240V RMS.  Negligible current flow / power dissipation as expected.




The bottom half: overvolted to 415V RMS.  The clamping is evident, significant current flows, and power dissipation is quite high.  If you hand-wave a typical 20mm disc having 200J withstand, and if that still applies within the same time constant over several line cycles, 200J / 370W = 0.54s endurance.  Granted, that's still a single event rating, so follow the lifetime curves accordingly (well, to whatever extent it's reasonable to extrapolate them out to 100ms or beyond, of course).

This isn't doing much as far as reducing voltage at the load; a lower rated MOV will clamp more, but also dissipate yet more in the process.  A larger resistor could be used, but will lose more in normal operation, which might affect operation of the relay (AC relays draw quite high current in the open position, allowing them to close quickly).

The resistance does do one other thing: if the elevated voltage causes the relay core (stator and/or armature) to saturate, it'll draw much more current (limited by resistance only), which may be more important to failure.  This might not take much overvoltage (say 20%?), so a MOV wouldn't be expected to help anyway, but the added resistance might give it a little longer before melting apart.

Overall, the sims show the difficulty of a shunting method.  Clearly a series limiter would be preferable here.  It's not at all impossible -- such circuits exist, I could make one -- but I don't think there's anything commonly available.

A much more alternative approach (and also one involving a lot more mucking around with the crane's controls) would be to replace the AC coils with DC (24V say?) and an industrial grade SMPS.  240-480V input ratings are readily available, not to mention surge ratings.  Ironically, the high-tech solution might indeed offer more robustness than the electromechanical version.  (Or, I mean, not really very ironic at all, kinda... more, goes to show, I guess?  Whatever. )

Tim

[Thermoelectric]

Thanks heaps for that. I'd have never thought to model it! Looks like a MOV would survive a little longer than I had expected, although not take many blows.

Along your lines of alternative solutions, I think my first point of call will be to ask the engineers if I can simply remove L2 from the control side of the bus bar. Change that to L1, and I no longer have these high current 415v faults to deal with (these could still come from the main supply rails but would have to bridge a few bars to get there, far less likely). Will still have the 230v short circuits caused by copper whiskers but that only results in lower energy transients which the existing MOV's may be able to handle.

I'm having a hard time understanding why they would use two separate phases like they have. These relays form a 2 channel safety circuit, so if either is de-energised, the crane won't run. They are potentially using the two phases as a very coarse phase failure indication, however they don't monitor L3 which seems like a pretty poor implementation. No discrete phase failure monitoring implemented elsewhere as far as I can see either.

Thanks for the input everyone! I'll report back if I get a bite from the engineers. Surge suppression like my original post was leaning towards seems more of a band-aid fix in hindsight, rather than eliminating/reducing the cause.