Why do tramways use 750V DC instead of an equivalent AC Voltage?

[Emyr]

It wouldn't be useful because you couldn't, without turning it back to AC, use a traditional wound transformer. But why more dangerous? 25kV of anything will fry you to a crisp if you get anywhere near it.

Arcs are inductive, which can create currents which overcome the ratings of DC switchgear. AC provides 0-crossings which help limit the effective life of an arc.

 

[455driver]

Is that why they make that weird high pitched sound, not the traction inverter warble but the screeching kind of sound from the PTSOL?

I dont know, I dont go near that horrible overhead stuff, much prefer my electric bits where I can kick them thanks!

 

[edwin_m]

I dont know, I dont go near that horrible overhead stuff, much prefer my electric bits where I can kick them thanks!

I'll let you do the kicking then, I trust your rubber boots are dry.

When I did the predecessor of PTS in 1987 I wasn't bothered about the OLE at all, as you'd have to go a long way out of your way to get too close to it. However the third rail seemed a lot more scary as just putting a foot in the wrong place could be fatal.

 

[JGR]

I've never heard of this and anyway the mains frequency in UK is 50Hz. DC is often said to be more dangerous than ac due to the hold on effect. If you get hold of an electric cable your muscles contract and you can't let go. With ac the voltage goes to zero every 20ms so giving you a slight chance to let go.

Realistically the zero crossing point point does not give you any meaningful chance to let go (at least at in the 50 or 60 Hz range).

As I understand it, AC is more dangerous to the body than DC at the same voltage or current, in part due to capacitative effects (the body has a lower impedance at 50 or 60 Hz than it does at DC, and the skin effect will not save you at this sort of frequency).

Arcs are inductive, which can create currents which overcome the ratings of DC switchgear. AC provides 0-crossings which help limit the effective life of an arc.

It's not the arcs themselves that are inductive. Any inductance would be in the line itself and/or the switchgear, I'd doubt that it'd be that significant TBH.
Arcs on the normal current path (i.e. at the shoe/pantograph) aren't a problem, they'll just add noise. Arcs in the wrong place, (temporary shorts) are just as bad as any other shorts.

 

[AM9]

Realistically the zero crossing point point does not give you any meaningful chance to let go (at least at in the 50 or 60 Hz range).

As I understand it, AC is more dangerous to the body than DC at the same voltage or current, in part due to capacitative effects (the body has a lower impedance at 50 or 60 Hz than it does at DC, and the skin effect will not save you at this sort of frequency).

DC is as has been said dangerous because the muscles are tensed and likely to lock onto the live conductor. AC on the other hand will cause the muscles to go into a shake mode which usually will cause contact to break, (providing the body is not resting as a dead* weight). At a frequency of 50Hz, there would be no opportunity to consciously remove during the zero-crossing points.
*No pun intended.
Body capacitance is very low, (for measurement purposes it is often regarded as 100pF, i.e. 1x10 to the power of -10) and that gives a capacitive reactance of 200Mohms. Therefore the current drawn from a 25kV supply would be in the order of a hundred micro-amps, a small jolt but survivable shock.
Body resistance is reckoned to be between 1000 and 100000 Ohms, mainly dependent on whether the skin is wet or dry. That means that direct contact to a 25kV supply would give a body current between 25 and 0.25 amps respectively. Both are way above lethal levels and significantly both well above any current caused by capacitance, so in the event of direct contact or proximity close enough to cause a flashover the capacitance is insignificant.
In the case of contact with a DC line, say 750V, a direct contact could result in a shock current between 0.75A and 7.5mA, both in the lethal range although it would be possible for some to survive if their skin was dry enough and the contact of short enough duration.

It's not the arcs themselves that are inductive. Any inductance would be in the line itself and/or the switchgear, I'd doubt that it'd be that significant TBH.
Arcs on the normal current path (i.e. at the shoe/pantograph) aren't a problem, they'll just add noise. Arcs in the wrong place, (temporary shorts) are just as bad as any other shorts.

Arcing between power supplies and legitimate loads is usually controlled by ensuring that contact is restored as soon as possible, e.g. 3rd rail breaks at pointwork, or by temporarily removing the load, i.e. balises at netral section of OHLE. The ABBs are a lot more important on the 1500VDC OHLE routes where repeated drawing arcs onto the neutral sections cause frequent wire failures, - not as bad as this one though:

https://www.youtube.com/watch?v=FivDcIHfguU

On this, the initial failure caused high currents to break the contact wire which then sagged and continued to short against the coach roofs. A spectacular example of why low voltage DC is difficult to protect against faults yet provide the high traction currents required. On ac power lines i.e. as with the national grid, a rapid change in the impedance of the circuit is detected and interpreted as a fault, then power is removed. Even if the line breaks and the current suddenly drops to zero, it can isolate the the line before the cable touches the ground. I'm not sure if that is true for 25kV OHLE.

 

[JamesRowden]

DC is as has been said dangerous because the muscles are tensed and likely to lock onto the live conductor. AC on the other hand will cause the muscles to go into a shake mode which usually will cause contact to break, (providing the body is not resting as a dead* weight). At a frequency of 50Hz, there would be no opportunity to consciously remove during the zero-crossing points.
*No pun intended.
Body capacitance is very low, (for measurement purposes it is often regarded as 100pF, i.e. 1x10 to the power of -10) and that gives a capacitive reactance of 200Mohms. Therefore the current drawn from a 25kV supply would be in the order of a hundred micro-amps, a small jolt but survivable shock.
Body resistance is reckoned to be between 1000 and 100000 Ohms, mainly dependent on whether the skin is wet or dry. That means that direct contact to a 25kV supply would give a body current between 25 and 0.25 amps respectively. Both are way above lethal levels and significantly both well above any current caused by capacitance, so in the event of direct contact or proximity close enough to cause a flashover the capacitance is insignificant.
In the case of contact with a DC line, say 750V, a direct contact could result in a shock current between 0.75A and 7.5mA, both in the lethal range although it would be possible for some to survive if their skin was dry enough and the contact of short enough duration.



Arcing between power supplies and legitimate loads is usually controlled by ensuring that contact is restored as soon as possible, e.g. 3rd rail breaks at pointwork, or by temporarily removing the load, i.e. balises at netral section of OHLE. The ABBs are a lot more important on the 1500VDC OHLE routes where repeated drawing arcs onto the neutral sections cause frequent wire failures, - not as bad as this one though:

https://www.youtube.com/watch?v=FivDcIHfguU

On this, the initial failure caused high currents to break the contact wire which then sagged and continued to short against the coach roofs. A spectacular example of why low voltage DC is difficult to protect against faults yet provide the high traction currents required. On ac power lines i.e. as with the national grid, a rapid change in the impedance of the circuit is detected and interpreted as a fault, then power is removed. Even if the line breaks and the current suddenly drops to zero, it can isolate the the line before the cable touches the ground. I'm not sure if that is true for 25kV OHLE.

I have found a reference at http://hypertextbook.com/facts/2000/JackHsu.shtml:

"At currents as low as 60 to 100 milliamperes, low-voltage (110-220 volts), 60-hertz alternating current traveling through the chest for a split second can cause life-threatening irregular heart rhythms. About 300-500 milliamperes of direct current is needed to have the same effect."

If one's body was close to creating a circuit directly, but with a small air gap, the voltages produced across one's body by a fallen over-head-line could be in this 110-220 volt range. Therefore making DC safer than AC in this particular situation.

 

[AM9]

I have found a reference at http://hypertextbook.com/facts/2000/JackHsu.shtml:



If one's body was close to creating a circuit directly, but with a small air gap, the voltages produced across one's body by a fallen over-head-line could be in this 110-220 volt range. Therefore making DC safer than AC in this particular situation.

If you get near enough to either 1500VDC or 25kV ac for flashover or even contact, I don't think survivability differences would have any impact on the choice of system. The bottom line is that subject to clearances from structures, 25kV OHLE is the choice for any new mainline railway electrification that isn't saddled with the need to connect to an existing DC or other system. Issues of safety from electrocution are virtually the same for both systems in terms of minimum distance and the minutae of whether an ac shock is more likely to kill than a DC one have no influence on selection. If you are that near, count yourself dead anyway.
The choice of ac versus DC at the same voltage is one of equipment necessary within the rolling stock versus that positioned at the feed substation.

 

[JamesRowden]

If you get near enough to either 1500VDC or 25kV ac for flashover or even contact

The OP asked about 750V DC Tramways.

There is also the fact that an AC current would not require flashover or contact to pass through a body since it can transfer through a capacitance across a thin insulator. While DC would only charge the capacitance and then not transfer any more power.

 

[JGR]

DC is as has been said dangerous because the muscles are tensed and likely to lock onto the live conductor. AC on the other hand will cause the muscles to go into a shake mode which usually will cause contact to break

Not sure about this bit. This is an interesting (if a tad morbid) read which suggests the opposite http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/#S2-7title

Body resistance is reckoned to be between 1000 and 100000 Ohms, mainly dependent on whether the skin is wet or dry. That means that direct contact to a 25kV supply would give a body current between 25 and 0.25 amps respectively. Both are way above lethal levels and significantly both well above any current caused by capacitance, so in the event of direct contact or proximity close enough to cause a flashover the capacitance is insignificant.
In the case of contact with a DC line, say 750V, a direct contact could result in a shock current between 0.75A and 7.5mA, both in the lethal range although it would be possible for some to survive if their skin was dry enough and the contact of short enough duration.

At 25kV you'd be likely to get dielectric breakdown of the skin which would significantly lower the overall impedance, making things even worse.
This could happen at 750V as well.

On this, the initial failure caused high currents to break the contact wire which then sagged and continued to short against the coach roofs. A spectacular example of why low voltage DC is difficult to protect against faults yet provide the high traction currents required. On ac power lines i.e. as with the national grid, a rapid change in the impedance of the circuit is detected and interpreted as a fault, then power is removed. Even if the line breaks and the current suddenly drops to zero, it can isolate the the line before the cable touches the ground. I'm not sure if that is true for 25kV OHLE.

Arguably it's the low voltage bit more than the DC bit which is the problem here, as there's not enough difference between the service and fault currents. Though the fact that DC arcs are less prone to self-extinguish doesn't help either.

On national grid lines it's 3 phase AC, and a cable-break failure would cause the line to become massively unbalanced amongst other things, which is presumably easy to detect (large currents in the ground tap, etc.). For high power lines, shutting things off quickly is necessary to stop the transformers at each end being wrecked by the imbalance.
As for 25kV OHLE, I'd doubt that you could do that, but shorts are easy enough to detect and shut off.

 

[AM9]

The OP asked about 750V DC Tramways.

There is also the fact that an AC current would not require flashover or contact to pass through a body since it can transfer through a capacitance across a thin insulator. While DC would only charge the capacitance and then not transfer any more power.

The OP was asking about 750VDC and ac. So for DC, it would be less than 2.5mm, for ac it would be 41% more to avoid the peaks. I don't think that would have any measurable bearing on the safety. Contact could easily result in a fatal shock depending on how well the body was earthed. As far as capacitance goes, not only is the body capacitance very low (about 120pF) but for current to flow the body would need to be very close to the conductor as capacitance and therefore the coupling current, is inversely proportionate to the distance between the two conducting surfaces. With a voltage as low as 750V (ac or DC) I can't imagine a circumstance where a person would accidentally be within the distance to suffer from capacitive coupling yet not get a shock by contact, unless they are on some death wish. So the only consideration is where the transition from local ac power to motor (or motor bus line) DC is made.

 

[JamesRowden]

The OP was asking about 750VDC and ac. So for DC, it would be less than 2.5mm, for ac it would be 41% more to avoid the peaks. I don't think that would have any measurable bearing on the safety. Contact could easily result in a fatal shock depending on how well the body was earthed. As far as capacitance goes, not only is the body capacitance very low (about 120pF) but for current to flow the body would need to be very close to the conductor as capacitance and therefore the coupling current, is inversely proportionate to the distance between the two conducting surfaces. With a voltage as low as 750V (ac or DC) I can't imagine a circumstance where a person would accidentally be within the distance to suffer from capacitive coupling yet not get a shock by contact, unless they are on some death wish. So the only consideration is where the transition from local ac power to motor (or motor bus line) DC is made.

I was thinking of a scenario in which a road traffic accident leads to the wires being knocked down which scrape the clothes of a passing pedestrian who has a shoe on one of the tram tracks.