# Re: Lightning & Surge protection

Home Electrical Engineering Forum General Discussion Lightning & Surge protection Re: Lightning & Surge protection

#13098
Spir Georges GHALI
Participant

@guest said:
i am studying short circuit calculation for power systems.pls send me the data for designing and selection of protective devices for sc

Dear ;

As we know all, at the end of life of any SPD a short circuit is happened between “ one phase or more ” and the “ earth point ”, therefor the SPD’s supplying should absolutely be disconnected, for this reason, we have 2 products can be used to disconnect the supplying once that short circuit happen, that are :

• Circuit Breakers “ C.B. ”: 2 ranges can be used “ MCB  &  MCCB ”, and it will better if they have only a Magnetic Protection.
• Fuses : they should be of “ Network Protection ” range.

After defining the used Earthing System for the network, and the SPD’s main specifications especially : the Number of Poles & the prospective Discharging Current, the more important specifications of the Protective Devices that should be defined are the following :

• Number of Poles : it should be exactly the same as the number of poles of SPD. Noting that if the SPD has a Neutral pole the protective device should absolutely has it.

• Nominal Current for C. B. : if they have only a Magnetic Protection, the usable Nominal Current are “ 25, 40, 63, 80, 100, 125, 160A ”, where the choice will be done depending on the discharge current’s value, as well as this value increase the nominal current’s value will be increased, but if C.B. have also a Thermal Protection, we define first the value of that protection ( see bellow ), and the Nominal Current will be defined accordingly ( depending on the manufactures’ ranges ).

• Breaking Capacities : this value can be defined after knowing the Maximum Short Circuit Current at the installation’s point, where “ Breaking Capacity  ≥  Iscmax ”. Noting that :

• The values of the Maximum Short Circuit Currents are normally calculated when we design any Electric Panel ( Electric Board ).
• If the SPD’s number of poles is “ 3P with or without Neutral ” the “ Iscmax ” is for a short circuit between “ 3 phases ”, but if the SPD is “ 1P+N ” the “ Iscmax ” is for a short circuit between “ 1 phase & neutral ”.

• Magnetic Sitting ( Short-time Sitting ) for Circuit Breakers :as at the end of life of SPD a short circuit between “ one phase or more and the earth point ” called an Earth Faull, we define the value of this protection depending on the CB’s type as follow :

• “ MCB ” : the type of this protection should be “ C curve ”.
• “ MCCB ” : the value of this protection will be defined after knowing the Earth Fault Current at the installation’s point, where “ Magnetic Protection  <  Id ”.

We define the value of the Thermal Protection either for Fuses or Circuit Breakers if they have also this protection ( called also Long-time Protection ) as follow :

• Thermal Sitting ( Long-time Sitting ) : if possible, we don’t need for C.B. an adjustable Thermal Protection, the usable settings are “ 20, 25, 32, 40, 50, 63, 80, 100, 125, and 160A ”, and the choice will be done depending on the discharging current’s value, as well as this value increase the sitting value will be increased. For example : for  discharging current “ 5kA ” we can use “ 20 or 25A ”, and for “ 65kA ” we can use “ 125A ”.

Rem. 1 : as the SDP should be replaced at the end of life, therefor we should know when should be, to know that we have 2 possibilities :

• By regular check-up of each installed SDP especially during the season of Lightning Strikes.
• By using SPDs equipped with an “ Auxiliary Contact ” that change his state at the end of life.

Rem. 2 : some manufactures produce SPDs  equipped with suitable “ Disconnectors ”, so, no need to use with those SPD’s any protective devices.

Rem. 3 : if the SPD has “ 2P ” ( 2 phases ), the same Breaking Capacity’s value for “ 3P ” can be used.