I have been asked:
Talking in terms of coil and contact resistance, how many contacts in series can make a relay drop (say Q series)??
It is a long time since I have been involved in New Works relay interlocking design, so don't know what current standards apply- hence would appreciate being directed to appropriate document. In the meantime I have given this response:
Firstly it is most unlikely that the relay will drop; what is far more probable is that it will fail to pick. That is because there is hysteresis- the pick up current of a relay is significantly greater (typically 150 percent) than the current required to keep the relay up.
The force required to pick the relay will require on such things a the number of contacts and the ratio of back contacts (whose pressure helps) and front contacts (whose pressure hinders), also crucially the coil resistance and number of turns. [Magnetic flux of coil given by "n" x "I"].
However giving some estimated typical figures, let us postulate that a relay will just pick when it has 40V across its coil connections and has a coil resistance of 800 ohms; we can therefore afford to drop 10V across the rest of the circuit. To be reliable, we need to make an allowance for the incoming power supply to be a little low, say 48V so we can therefore afford to drop 8V across the various contacts (assuming that the drop along the wires and spade connections is negligible in comparison). We have 50mA flowing through them, so that gives a total of 160 ohms. Contacts generally will have a low resistance, probably I guess rather less than 1 ohm.
The problem is that sometimes high resistance contacts develop (say 10 ohms) and too many of these in series mean the end function won't pick. Also when establishing contact, the higher voltage available as the surfaces approach each other, the better the chance of establishing good electrical contact as it may have to breakdown a thin surface film of contamination. The relay coil will present an inductive load; this will mean the build up of current will not be instantaneous.
Back contacts are especially prone to high resistance, particularly those used as "down-proving" as it is unlikely that they will ever switch the load- this causes wear but on the plus side ensures that the switching surface gets some cleaning as a result.
You can see that the circuit will continue to work even in the presence of several high resistance contacts of that sort of order, however if one contact deteriorates so much that its resistance gets higher than around 100ohms then we would be in trouble if there are also some others which are also not too good.
Actually I think the main determinant in selecting a maximum number of contacts is how long it would take the maintenance technician to locate the fault in a circuit; the more contacts in the circuit the longer it will take to go prodding around with a voltmeter to find where the voltage is being lost. Bear in mind that until the end relay is picked up, then a high input impedance meter will actually "see" effectively full voltage through a high resistance contact (as very little current is yet flowing). When a subsequent contact makes (i.e another relay was initially in the wrong state when the fault tracing commenced) to complete the link between the faulty contact and the end function, current will now attempt to flow across the high resistance of the previously checked function and suddenly voltage is lost across it. The technician could already have thought they had discovered this contact was ok and is directing their attention elsewhere.
Therefore many railways will specify what they think is a reasonable number of relay contact is in series in any end function. I seem to remember that the Western Region worked to a limit of 25 contacts, of which 5 could be back contacts, but the BR national standard was quite a bit lower, I think 10 or 12. I do not know whether NR's Design Handbook or any other documentation specifies a "good practice" limit.
I think however that a low (perhaps it was 5) limit was set for the number of point detection contacts which could be summated into an SSI points TFM input. Not sure where this came from, but certainly there are commissioned sites with double that number which haven't experienced any problems- also note that SSI input is a low voltage (5V) pseudo-random square wave code and the TFM input is probably higher input impedance than a relay, so there is little current available for "contact wetting" and hence more susceptible to high resistance contacts than a relay would be.
Hope someone can add to this........
Section 6.2.4 of 11600 states the max no of contacts as 20.
Talking in terms of coil and contact resistance, how many contacts in series can make a relay drop (say Q series)??
It is a long time since I have been involved in New Works relay interlocking design, so don't know what current standards apply- hence would appreciate being directed to appropriate document. In the meantime I have given this response:
Firstly it is most unlikely that the relay will drop; what is far more probable is that it will fail to pick. That is because there is hysteresis- the pick up current of a relay is significantly greater (typically 150 percent) than the current required to keep the relay up.
The force required to pick the relay will require on such things a the number of contacts and the ratio of back contacts (whose pressure helps) and front contacts (whose pressure hinders), also crucially the coil resistance and number of turns. [Magnetic flux of coil given by "n" x "I"].
However giving some estimated typical figures, let us postulate that a relay will just pick when it has 40V across its coil connections and has a coil resistance of 800 ohms; we can therefore afford to drop 10V across the rest of the circuit. To be reliable, we need to make an allowance for the incoming power supply to be a little low, say 48V so we can therefore afford to drop 8V across the various contacts (assuming that the drop along the wires and spade connections is negligible in comparison). We have 50mA flowing through them, so that gives a total of 160 ohms. Contacts generally will have a low resistance, probably I guess rather less than 1 ohm.
The problem is that sometimes high resistance contacts develop (say 10 ohms) and too many of these in series mean the end function won't pick. Also when establishing contact, the higher voltage available as the surfaces approach each other, the better the chance of establishing good electrical contact as it may have to breakdown a thin surface film of contamination. The relay coil will present an inductive load; this will mean the build up of current will not be instantaneous.
Back contacts are especially prone to high resistance, particularly those used as "down-proving" as it is unlikely that they will ever switch the load- this causes wear but on the plus side ensures that the switching surface gets some cleaning as a result.
You can see that the circuit will continue to work even in the presence of several high resistance contacts of that sort of order, however if one contact deteriorates so much that its resistance gets higher than around 100ohms then we would be in trouble if there are also some others which are also not too good.
Actually I think the main determinant in selecting a maximum number of contacts is how long it would take the maintenance technician to locate the fault in a circuit; the more contacts in the circuit the longer it will take to go prodding around with a voltmeter to find where the voltage is being lost. Bear in mind that until the end relay is picked up, then a high input impedance meter will actually "see" effectively full voltage through a high resistance contact (as very little current is yet flowing). When a subsequent contact makes (i.e another relay was initially in the wrong state when the fault tracing commenced) to complete the link between the faulty contact and the end function, current will now attempt to flow across the high resistance of the previously checked function and suddenly voltage is lost across it. The technician could already have thought they had discovered this contact was ok and is directing their attention elsewhere.
Therefore many railways will specify what they think is a reasonable number of relay contact is in series in any end function. I seem to remember that the Western Region worked to a limit of 25 contacts, of which 5 could be back contacts, but the BR national standard was quite a bit lower, I think 10 or 12. I do not know whether NR's Design Handbook or any other documentation specifies a "good practice" limit.
I think however that a low (perhaps it was 5) limit was set for the number of point detection contacts which could be summated into an SSI points TFM input. Not sure where this came from, but certainly there are commissioned sites with double that number which haven't experienced any problems- also note that SSI input is a low voltage (5V) pseudo-random square wave code and the TFM input is probably higher input impedance than a relay, so there is little current available for "contact wetting" and hence more susceptible to high resistance contacts than a relay would be.
Hope someone can add to this........
Section 6.2.4 of 11600 states the max no of contacts as 20.
PJW