Current Limiting PD's

[RodneyDixon]

All,

I see many LV protective devices come with current limiting abilities in the form of a curve from which one can determine the limited current based on the initial short circuit current. Those involved studies, from what I can see the protective libraries in commercial software do not have this incorporated. What do you guys do in these cases?

Could utilise a reactor to limit fault to desired level could be one approach?

Interested in anyone's thoughts on this.

Best,

RD

 

[dpc]

You'll have to be more specific. In the US, "current-limiting" means that the fault is cleared with the first half cycle. This only occurs for a range of available fault current. If you are wanting to protect downstream protective device via an upstream current-limiting device, the only acceptable approach is to apply a tested combination. There is no accepted way of calculating the amount of let-through current.

 

[RodneyDixon]

dpc thanks you for input - my query relates to Europe.

Looking at Siemens Simaris software (PD Curves) a current let-through curve is provided (see attached with random values plotted on the curve). This was what my query was referring to.

Any thoughts on these?

Thanks
Link

 

[dpc]

Again, it will depend on what you are trying to determine. We have similar "let-through" curves in the US from fuse manufacturers, but the use of these curves is deprecated when trying to apply a current-limiting fuse upstream of a circuit breaker to get around the short circuit rating limitation of the breaker. As noted earlier, the fuse and the breaker must be a tested combination. There is no accepted method of calculating the fault current that the downstream breaker will see. These let-through curves may have some value in other situations, but it will be application-specific.