Third rail maximum current

[ewsclass]

Just wondering what the maximum permitted current draw per unit is on third rail?

 

[py_megapixel]

Don't quote me on this but doesn't it vary based on area? (as in some particularly high draw units won't get cleared for routes with weaker power supplies).

Welcome to the forum by the way!

 

[GRALISTAIR]

Just wondering what the maximum permitted current draw per unit is on third rail?

Welcome to the forum. I will try and find the answer for you. As the poster above says, it does vary. Bournemouth to Weymouth is lower than Clapham Junction for example .

 

[swt_passenger]

Nearly all modern DC powered units are software limited and draw nothing like their theoretical calculated current. For example, it’s been explained here before that 450s can only draw 55% of their full power, so that’s about 1500A. A 444, being 5 x 23m is obviously bigger and can draw 70%, getting towards 1900A.

But many trains run in 12 or 10 car sets, not forgetting some 12 car fixed formations.

As said, some areas of the network such as Poole to Weymouth will only allow single units, say single 444s to operate, due to tighter supply limits. In most cases Network Rail’s equipment is operating with very little margin above the present timetable. Any time a significant increase in the timetabled frequency or normal train length is planned, the response is usually to provide just enough power, a recent example being the successive increase over a few years to allow all day 10 car services at 4 tph to Reading...

 

[Driver2B]

I believe the trains are fitted with 800 amp fuses, so less than that, I presume.

 

[hexagon789]

I read somewhere that 375/377s can draw a maximum of 1778 amps on 3rd rail

 

[hexagon789]

I believe the trains are fitted with 800 amp fuses, so less than that, I presume.

Would that be per motor?

 

[swt_passenger]

Would that be per motor?

Per shoe or maybe bogie?

 

[aleggatta]

I read somewhere that 375/377s can draw a maximum of 1778 amps on 3rd rail

the 387 line of route states shoe fuses are rated to 1133A, and one shoe fuse will provide propulsion for a 4 car unit. There is no reason for 4 car 377/375's to have any more or less maximum current draw set up, however that's not to say that 1778 is not a valid figure for 5 car 376/377 units? (I know I don't have that detail available to me.

I am slightly confused however, as I know the High Speed Circuit Breakers on electrostars are generally rated to have a tripping current of 1200A, but the relatively slow blow nature of the ribbon fuse probably negates it failing before the HSCB opens, but that does still seem to leave me with more questions than I started with...

 

[swt_passenger]

Someone posted a circuit diagram of a 377 a few months back, it shows 8 fuses, ie one for each individual shoe, and 3 x HSCB, so each HSCB should only see a third of the overall current, well within 1200A.

I can’t see full power being available through one shoe though?

 

[Driver2B]

Would that be per motor?

Per shoe or maybe bogie?

On 444s and 450s, I think there are two 800 amp fuses on the power train line? (One 800 amp fuse on each driving coach?)

When working in multiple, the power train line is not connected (only the 110v supply is connected between units), so would that mean that two units in multiple would draw double and three in triple would draw triple?
(I'm certainly no electrician!)

 

[swt_passenger]

On 444 and 450s, I think there are two 800 amp fuses on the power bus/train line? (One 800 amp fuse on each driving coach?)

I’ve never seen that level of detail I’m afraid. It does seem a bit low to me, in terms of the usual sorts of currents flying around.

 

[Driver2B]

I can’t see full power being available through one shoe though?

I believe each shoe can draw full power. The main advantage of having more shoes is that there is less risk of gapping (ie, the train is less likely to find itself with no shoes on a conductor rail and then, in essence, unplugged).

 

[aleggatta]

Someone posted a circuit diagram of a 377 a few months back, it shows 8 fuses, ie one for each individual shoe, and 3 x HSCB, so each HSCB should only see a third of the overall current, well within 1200A.

I can’t see full power being available through one shoe though?

that was me, and as part of my training we were informed that a unit could be fully powered and running up off of one shoe, so if it needed to in fault conditions, it could safely.

That being said, I'm gonna do some simple maths....

Traction motors on an e* are 200KW each... there are six in a four car unit, so that's 1200KW. simple maths makes that 160A at 750V on traction motors, so 1133A might be a correct figure with safety margin included, I can't find any detailed info on the loadings of the auxiliary supplies to support this hypothesis...

(if my maths is wrong, I do apologise!)

 

[swt_passenger]

that was me, and as part of my training we were informed that a unit could be fully powered and running up off of one shoe, so if it needed to in fault conditions, it could safely.

That being said, I'm gonna do some simple maths....

Traction motors on an e* are 200KW each... there are six in a four car unit, so that's 1200KW. simple maths makes that 160A at 750V on traction motors, so 1133A might be a correct figure with safety margin included, I can't find any detailed info on the loadings of the auxiliary supplies to support this hypothesis...

(if my maths is wrong, I do apologise!)

You’ve lost a decimal place I think, it should be 1600A

 

[Philip Phlopp]

that was me, and as part of my training we were informed that a unit could be fully powered and running up off of one shoe, so if it needed to in fault conditions, it could safely.

That being said, I'm gonna do some simple maths....

Traction motors on an e* are 200KW each... there are six in a four car unit, so that's 1200KW. simple maths makes that 160A at 750V on traction motors, so 1133A might be a correct figure with safety margin included, I can't find any detailed info on the loadings of the auxiliary supplies to support this hypothesis...

(if my maths is wrong, I do apologise!)

Don't assume the voltage is always 750V though...

 

[aleggatta]

You’ve lost a decimal place I think, it should be 1600A

quite possibly, brain is a little tired!

Don't assume the voltage is always 750V though...

some would simply say never assume voltage at all! as long as the over-voltage protection doesn't kick in you're alright!

 

[AM9]

Don't assume the voltage is always 750V though...

... and don't assume that an 800A fuse will blow with a little more than 800A running through it. Fuses can carry in excess of their nominal rated current depending on the duration. If say a train was traversing complex pointwork with several condictor rail breaks, there could be short periods where three of the four shoes (on one side of a four car unit) are breifly out of contact with the supply. Thus all of the current for the unit's motors would be passing through that shoe and its wiring connection to the DC bus. A 800A fuse would be able to carry that current for a second or two before the temperature rose enough to blow it, until another shoe was able to share the current.

 

[big all]

also worth mentioning the shoe and train line fuse must cover not only all traction and hotel requirements on the unit but also a margin for when bridging between two substation or supplies with an imbalance and drawing additional current [power]from one to the other ??

 

[DerekC]

The OP asked for the maximum current draw per unit. My guess is that the prize would go to either a 12-car Class 700 or the old DC-capable Eurostar. The class 700 packs 5MW which would equate to over 6000A at 750V if it was allowed to pull that much. I can't recall what the maximum DC current is - I have a feeling it might be around 4500A. The Eurostar Class 373 is stated to have a limit of 3.4MW in DC mode, which would be about the same. I know it needed special reinforcement.

 

[MarkyT]

The OP asked for the maximum current draw per unit. My guess is that the prize would go to either a 12-car Class 700 or the old DC-capable Eurostar. The class 700 packs 5MW which would equate to over 6000A at 750V if it was allowed to pull that much. I can't recall what the maximum DC current is - I have a feeling it might be around 4500A. The Eurostar Class 373 is stated to have a limit of 3.4MW in DC mode, which would be about the same. I know it needed special reinforcement.

Class 92s are rated at 4MW on DC, and their Channel tunnel routes were upgraded to 750V high current specification supplies, so that suggests they might be able to draw up to 5,333 Amps. ISTR the 'high current' work involved doubling the number of substations by converting former so-called 'track paralleling' (TP) huts that previously alternated with substations along the routes into additional full-blown substation feeds with their own 33kV transformers.

 

[swt_passenger]

... and don't assume that an 800A fuse will blow with a little more than 800A running through it. Fuses can carry in excess of their nominal rated current depending on the duration. If say a train was traversing complex pointwork with several condictor rail breaks, there could be short periods where three of the four shoes (on one side of a four car unit) are breifly out of contact with the supply. Thus all of the current for the unit's motors would be passing through that shoe and its wiring connection to the DC bus. A 800A fuse would be able to carry that current for a second or two before the temperature rose enough to blow it, until another shoe was able to share the current.

That’s a fairly reasonable explanation, I think my issue was with one shoe for a reasonably continuous period.

 

[hexagon789]

On 444s and 450s, I think there are two 800 amp fuses on the power train line? (One 800 amp fuse on each driving coach?)

When working in multiple, the power train line is not connected (only the 110v supply is connected between units), so would that mean that two units in multiple would draw double and three in triple would draw triple?
(I'm certainly no electrician!)

Which would be 1600amps per train which seems too low if 1500amps is ~55% full draw.

 

[hexagon789]

the 387 line of route states shoe fuses are rated to 1133A, and one shoe fuse will provide propulsion for a 4 car unit. There is no reason for 4 car 377/375's to have any more or less maximum current draw set up, however that's not to say that 1778 is not a valid figure for 5 car 376/377 units? (I know I don't have that detail available to me.

I am slightly confused however, as I know the High Speed Circuit Breakers on electrostars are generally rated to have a tripping current of 1200A, but the relatively slow blow nature of the ribbon fuse probably negates it failing before the HSCB opens, but that does still seem to leave me with more questions than I started with...

I'm afraid it's like many pieces of information I pick-up, after a time of never having needed to use the info you forget where it came from or indeed the precise nature of that data. I think it was a 4-car unit and I believe on 3rd rail the draw was permitted to 25mph but I couldn't swear to it with 100% certainty

 

[hexagon789]

Per shoe or maybe bogie?

Didn't really think of those options

 

[swt_passenger]

Didn't really think of those options

I was thinking back to the 458/5 fire at Windsor, where the shoe cables were commoned together on or near the bogie, but they had no inline fuse protection for the cables to the circuit breaker anyway. There’s probably as many permutations as possible across the older fleets.

They calculated that when accelerating away from Windsor during the incident the unit in question would have been drawing 1900A through one shoe for a very short time, because of a long conductor rail gap.

 

[notadriver]

Looking in the Mitrac I saw 2000 hp for a 4 car 375 using the formula tractive effort (For a coach times 3 traction packages) * speed divided by 375

 

[AM9]

The OP asked for the maximum current draw per unit. My guess is that the prize would go to either a 12-car Class 700 or the old DC-capable Eurostar. The class 700 packs 5MW which would equate to over 6000A at 750V if it was allowed to pull that much. I can't recall what the maximum DC current is - I have a feeling it might be around 4500A. The Eurostar Class 373 is stated to have a limit of 3.4MW in DC mode, which would be about the same. I know it needed special reinforcement.

I think that a 3rd rail circuit is rated at a maximum of 10,000A, so limiting trains to around 4000 to 4500A would cover the maximum likely occupancy of a length over which the voltage drop would be acceptable. Any higher current limit would make fault detection unreliable. This is the main weakness of a low voltage traction supply system with modern main line power demands, even if all the human safety issues are ignored.

 

[swt_passenger]

I think that a 3rd rail circuit is rated at a maximum of 10,000A, so limiting trains to around 4000 to 4500A would cover the maximum likely occupancy of a length over which the voltage drop would be acceptable. Any higher current limit would make fault detection unreliable. This is the main weakness of a low voltage traction supply system with modern main line power demands, even if all the human safety issues are ignored.

The RAIB report I was looking at earlier re the Windsor incident, explained that the end of that branch was fed through two 7000A circuit breakers, one for each track, and this meant the pretty massive train earth fault was undetectable for the reasons you give.

 

[Philip Phlopp]

The RAIB report I was looking at earlier re the Windsor incident, explained that the end of that branch was fed through two 7000A circuit breakers, one for each track, and this meant the pretty massive train earth fault was undetectable for the reasons you give.

It wouldn't be third-rail if the rolling stock wasn't setting itself on fire...