Min & Max Signal Spacing

[NJK]

Dear Members,

In 3 aspect signalling, the maximum signal spacing "d" is calculated by using following formula

d = ((Vh x Vt) - (Sd + O + L)) / 2

some times the calculated value "d" is exceeding the value 1.5BD. If the value d exceeding 1.5BD then risk assessment is required.

My doubt is whether I have to use 1.5BD or d for maximum signal spacing.

Kindly clarify.

Regards,
NJK






'

[PJW]

The formula giving the MAXIMUM spacing is from the HEADWAY constraint.
You also need to satisfy the OVER-BRAKING constraint.
So it is not a question of which to respect; there are two constraints and you must satisfy BOTH. In any particular situation, you use the limiting factor- the one which is the SMALLER of the two MAXIMUMS.

If can't achieve whatever this MAXIMUM is plus the MINIMUM derived from BRAKING, then use 4 aspects instead.

This is the sort of question that can be answered from an iPad; today I am travelling to Stratford Olympic Park wearing a magenta-pink tabbard with a mission to get the hoards to use West Ham station instead!

I am conscious that I still owe other replies, but they need to wait until I have suitable iopportunity to respond, unless someone else takes up the reins!

Dear Members,

In 3 aspect signalling, the maximum signal spacing "d" is calculated by using following formula

d = ((Vh x Vt) - (Sd + O + L)) / 2

some times the calculated value "d" is exceeding the value 1.5BD. If the value d exceeding 1.5BD then risk assessment is required.

My doubt is whether I have to use 1.5BD or d for maximum signal spacing.

Kindly clarify.

Regards,
NJK






'

[Jerry1237]

d gives the maximum distance to meet the criteria for operability.

1.5BD is rules based and should be an exceptional deviation from normal practice.

Remember, each country has differening rules. It should be stated as an assumption what each acyonym means (i.e. BD, d) and where limits are used (1.5, i.e. UK mainline practice).

Also, mininimum signal spacing is a safety distance. Maximum is an operational figure. The distinction between the two is very important to understand and relay to the examiner.