The Surge can be termed as a temporary over voltage which brings with it a large amount of destructive energy which can damage the sensitive electrical & electronic equipments.

With the increasing degree of sophistication in all the electronic components being used in industry as well as house hold use, these equipments are very susceptible to over voltages and as such they need to be protected against the over voltage surges.

All the existing protecting devices like MCB, ELCB etc are current sensitive devices. Ie., they get activated based on a fault current. These devices are triggered when the fault current persists for a duration of more than 3 to 20 milli seconds. However, the surge typically lasts for a few micro seconds only during which time it causes all the damage.

Hence the conventional devices like MCB, ELCB fuses etc are not effective in protection against surges. In view of the above, a special equipment called surge protection device which has a response time of two nano seconds is required to be installed to protect against the surges.

The SPDs are voltage sensitive devices which are connected across the circuit. They are triggered by a transient over voltage and have a response time of nano seconds.

Surges are transient over voltages which can be the result of a spark over or fault in electrical system due to switching or failure of a movable contact in an electrical switching equipment.

The surges are also caused by lightning which brings with it enormous amount of destructive energy.

The lightning striking at a distance from the given equipment can cause failure of the said equipment due to

1)Galvanic coupling, (2) Capacitive coupling, (3) Inductive coupling

Surges are broadly categorized as switching surges and lightning surges depending on the nature of the origin. The lightning surges are categorized as Class-B or Type-1 surges and switching surges are categorized as Class-C or Type-2 surges.

Some of the popular technologies used in SPDs are the following:

i) Gas discharge tubes
ii) Metal oxide varistors
iii) Spark gap
iv) Zener diode
v) Silicon Avalanche diodes

Among these, gas discharge tubes, metal oxide varistors and spark gap are the most popular types of surge arresters.

Generally, the SPDs installed on the electrical Power circuit are connected in parallel and hence will be independent of load (line) current.

The selection of SPDs at a given location depends on:

a) The topography of the location in terms of lightning strikes.
b) Susceptibility to switching surges in terms of proximity to large loads / connected to rural feeder etc.
c) Sensitivity of the equipment connected to the circuit.

Normally SPDs with Class-B & C are installed at the main panels and SPDs with Class-C rate are installed at the sub panels. The ratings for class-B, SPDs shall be a minimum of 12.5 KA Iimp.

For the Class-C (Imax rating), it should be , a minimum of 40 kA at the sub panels.

For the Class-D SPDs which are installed near sensitive equipment, the criteria shall be the residual voltage or the protection level which should be lower than the temporary over voltage withstanding capacity of the protected equipment.

Since the SPDs are always connected in parallel to the electrical circuit, during the normal course it is not possible to know whether they are in working condition or not. Hence it is necessary to have a local and remote indication using electrical contact in order to know and monitor the healthiness of the SPD.

It is not necessary to have a separate or dedicated grounding for SPDs. The SPDs should always be connected to the general earth grid.

All SPDs work by diverting the excess energy to the ground. Hence it is imperative that the earthing system connected to the SPD must be in efficient working condition so that the surge current can flow to the earth without any hindrance.

Both the TVSS and SPDs are based on the MOV technology only. The main difference between them are the configuration in which the MOVs are used. While the SPD uses a maximum of 3 or 4 higher capacity MOVs in parallel, the TVSS normally uses a large number of smaller capacity MOVs in parallel.

The SPDs are based on the IEC standards while the TVSS are based on IEEE and UL standards for design , testing and performance criteria.

SPDs are certainly available for every circuit like telephone lines, data lines, Ethernet lines, signal lines etc. In all these circuits SPDs are connected in series.