I've found this below
ANALOGUE EXTN LINE LIMITS / DIGITAL LOOP RESISTANCE.
For 500 set:
REN 1/2...........MAX LOOP RESISTANCE......1000 OHMS
REN 3.............MAX LOOP RESISTANCE...... 900 OHMS
REN 4..............MAX LOOP RESISTANCE...... 600 OHMS
REN 5..............MAX LOOP REISTANCE......... 350 OHMS
For 500 set:
ASSUMING YOU ARE USING 0.5MM COPPER CABLE THE
FOLLOWING IS A GUIDE TO THE MAXIMUM LENGTH OF
CABLE TO BE USED:-
REN 1/2..............11.4 KILOMETRES
REN 3.................10.25 KILOMETRES
REN 4.................6.8 KILOMETRES
REN 5.................4 KILOMETRES
PLEASE NOTE :: THESE FIGURES ARE FOR THE LINE LOOP
SO FOR EXAMPLE,AN EXTN WITH A REN OF 5 SHOULD NOT
BE FITTED MORE THAN 2 KILOMETRES FROM THE EQUIPT.
R = p.l/a
where R = resistance
p = resistivity ( copper is 17.2 x 10 to the power -9 )
l = length in metres
a = cross sectional area of the conducter ( metre sq )
For Aries type set:
Digital set loop resistance must be below 175 ohms. (approx 3000 ft)
All the best
Firebird Scrambler
Meridian 1 / Succession and BCM / Norstar Programmer in the UK
If it's working, then leave it alone!.