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Topic Title: Surge Protective Device for Backup Generator Supply
Topic Summary: Surge Protective Device
Created On: 17 April 2018 07:58 AM
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 17 April 2018 07:58 AM
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ietkaushika

Posts: 27
Joined: 14 January 2018

Hi Everyone,

For 250A power distribution panel with manual changeover switch, I have to add surge protective device. One incoming supply for power panel come from public utility and other incoming supply come from backup generator. Backup generator and power panel will be located in same building.

I have to use Type 1 SPD at the incoming supply from public utility.

My question is should I use surge protective device at the incoming supply from indoor backup generator for power panel? (What is the recommendation in British Standard?)

If I have to use SPD at the incoming supply from indoor backup generator, what is the type of SPD should I use?

Thanks & Regards,
Kaushika
 17 April 2018 10:49 AM
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AJJewsbury

Posts: 16834
Joined: 13 August 2003

BS 7671 doesn't give you a simple yes/no answer to that kind of question. SPDs are usually used to mitigate either overvoltages of atmospheric origin (i.e. due to lightning) or overvoltages due to switching. As the designer you'd have to work out if either apply to your generator supply. I might guess that a generator within the same building might be pretty immune to lightning, but there might possibly be some rare situations where you might want to consider that still, as for switching operations you'd have to look at the rest of your design for that. (I might have a think about possible disconnection of large inductive loads at a point when the transfer switch has disconnected the grid supply but not yet connected the generator supply for instance).

- Andy.
 17 April 2018 11:17 AM
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ietkaushika

Posts: 27
Joined: 14 January 2018

Dear Andy,

What are types inductive loads that you have meant can lead to create transient over voltages? Can you give some examples?

Thanks & Regards,
Kaushika
 17 April 2018 12:16 PM
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broadgage

Posts: 2801
Joined: 07 August 2007

Electric motors are highly inductive, and as a rough guide the greater the capacity of the motor, the greater the surge that may be produced during switching.
Transformers also, especially relatively large ones as might be found in welding sets, X ray machines, industrial battery chargers and the like.

Inductive lamp ballasts are generally small individually, but large numbers can be problematic.
 17 April 2018 01:52 PM
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ietkaushika

Posts: 27
Joined: 14 January 2018

Dear All,

I would like clear my point of view regarding this phenomena with basic theories as follows. Please help me if I am wrong.

When power disconnection is happened, stored magnetic flux inside inductive load try volatile withing small time. Due to this action, high voltage can be created in highly inductive loads and can be propagated towards other loads and incoming supply. If inductive loads are connected in series, higher transient voltage can be created and propagated towards other load and incoming.

Also I would like to ask some further questions regarding this.

1. Is there any industrial practice to use SPD near highly inductive load ends?

2. Is there any industrial practice to use SPD to protect back up generator from transient overlarge that is caused due to inductive loads?

3. What is the best location of main power circuit to fix SPD ? (at the incoming of utility supply, at the incoming of back up generator, outgoing of main breaker (to avoid reverse surge from loads))

Thanks & Regards,
Kaushika
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