LOW VOLTAGE LIMITERS FOR RAILWAY APPLICATIONS

LOW VOLTAGE LIMITERS FOR RAILWAY APPLICATIONS

P400G and P250G are a new types of low voltage limiters (voltage limiter devices type 1 .... VLD-F) based on requirements of EN 50122-1, which are designed to protect the non-live parts of metallic structures in DC ev. AC traction power supply systems. They are used to effectively protect persons who may come into contact with these parts during lightning strikes or a defect of the tractive overhead line. The limiter has a high internal resistance when voltage on it is less than the nominal level its spark over voltage UVDC and becomes conductive when this level is exceeded.

In case of failure arising from a contact between live parts of the power system and traction conductive portion unintentionally associated with return path limiter protects illegal contact voltage that becomes conductive and causes the power is turned off. According to EN 50122-1, this type of limiter is recommended primarily for connection between the protected area and return circuit in overhead line areas (or pantograph areas) that may be in contact with live conductors or damaged current collector, then the support structures pylons which can become live due to a fault isolation. After re-applied voltage drops below of its nominal level spark over voltage UVDC the limiter returns to the non-conducting state again.  
Functional part P400G (P250G) is made using a special gas-filled discharge tube (GDT) with a reduced response time of 20 nsec, rated up to 3 consecutive 50kA (10/350) lightning strikes. The outer insulating cover is made from light blue silicone rubber,  which is hydrophobic and has excellent resistance to weathering, pollution and UV radiation. The product is supplied in an assembly with an integrated bracket, which allows direct mounting of the limiter on a protected metal costruction (column, wall or flange).The recommended mounting position is vertical bracket up. Mountig bracket, connecting bolts and nuts are made from stainless steel suitable for conductors with cross section of 16 to 50 mm2 Fe (Cu).

Limiter is installed directly to a protected building structure (using two M12 bolts) so that, if activated, a conductive connection between this structure and the reverse circuit is created. The limiter can be activated either by flash current or by the current due to the unintentionaly contact of the protected metal construction with the downstream traction line. In this case a potencial difference of more than 400V occurs between these parts (applies to P400G) or 250V (applies to P250G). The built-in GDT immediately ignites (the response time is typically 20 nsec) and for a transitional period of both parts electrically interconnects (typical internal resistance initialized lightning arrestor P400G (P120G) is 0,001 ÷ 0,002 Ω). The duration of this transient process is automatically broken up by equalizing the potential between the protected structure and the reverse circuit  when the GDT automatically switches off due to the of its gas filling.


Advantages:

1) Products P400G ( P250G ) are maintenance-free, have very little reaction time (20nsec), high insulation resistance (>1GΩ) and high protective ability. Next is this product resistant against to harsh treatment, acid rain and mechanical damage.

2) To perform diagnostics it is recommended to use measuring instrument G-TESTER (supplier ACER VOLTAGE s. r. o.). This device allows  to control the one hand, the DC spark over voltage of the built GDT,  then the value of the insulation resistance Risol (from 25VDC up to 1000VDC).