LOW VOLTAGE LIMITERS FOR RAILWAY APPLICATIONS

LOW VOLTAGE LIMITERS FOR RAILWAY APPLICATIONS

P60G and P120G 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 P120G (P60G) is made using a special gas-filled gas discharge tube (GDT) with shortened response time 20 nsec, rated for up to 3 consecutive strikes of lightning current 50kA (10/350). On the outer insulating cover is used a light blue silicone rubber.  Working chamber of built GDT is equipped with technically sophisticated fail-save mode hardware that provides an automatically transition to short-circuit mode in the event of longtime overloads above 500A DC (AC) (this state is non-reversible). On the outer insulating cover is used a blue silicone rubber, which is hydrophobic and has excellent resistance against weathering, pollution and UV rays. Mountig bracket, connecting bolts and nuts are made from stainless steel, suitable for connection of conductors with cross section 16-50 mm2 Fe (Cu). The product is delivered with an integrated bracket, allowing direct mounting of P120G (P60G) to the protected metal structure (pillar, wall or flange). Recommended mounting position is vertical bracket up.

Limiter is installed directly to a protected building construction (using two bolts M12), so that in the event of its activation was generated a conductive connection between this structure and the return path. The limiter can be activated either by lightning current ev. current resulting from contact protected metallic structure with a fallen overhead line. In such a case occurs between these parts of the occurrence of a potential difference of more than 120V (valid for P120G), ev. 60V (valid for P60G). Built-in GDT ignites instantly (response time is typically 20 nsec) and for a transitional period of both parts electrically interconnects (typical internal resistance initialized P120G (P60G) is 0,001 ÷ 0,002 Ω). The duration of this transient process is automatically broken up by potential equalization between the protected structures and a return path, when it comes to an automatic switching off GDT due to recombination its gas filling.
 

Advantages

1) P60G and P120G limiters are maintenance free, have very little reaction time (<20nsec), high insulation resistance (> 2GΩ), high protective capacity and are equipped with fail-save hardware, which makes their diagnostic easy. They are resistant 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 (UVDC) of the buit GDT, then the insulation resistance value Riso (from 25VDC up to 1000VDC).