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Topic Title: 18th DPC - Selectivity between RCDs
Topic Summary: discrimintion between S types?
Created On: 10 August 2017 01:07 PM
Status: Read Only
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 10 August 2017 01:07 PM
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AJJewsbury

Posts: 15634
Joined: 13 August 2003

536.4.1.4 says:
Selectivity in case of residual currents, as shown in Figure 536.2, is given under the following conditions:
- the upstream RCD is of selective type (type S or time-delayed type with appropriate time delay setting), and
- the ratio of the rated residual operating current of the upstream RCD to that of the downstream RCD is at least
3:1.

Which says to me if I have a 100mA S type downstream and and a 300mA S type upstream I should get discrimination - but I thought (e.g. looking at table 3A in the current edition) the delays on S-types were much the same regardless of residual rating (just enough to discriminate with a instantaneous RCD downstream). Am I wrong, does the delay on S-type RCDs increase with residual rating?

- Andy.
 10 August 2017 02:16 PM
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burn

Posts: 236
Joined: 06 June 2003

I think the critical words are "with appropriate time delay setting", meaning with a 3:1 current ratio, you still have to adjust the setting of the upstream device higher than the downstream one.
Which all means you can only do it if the devices in question have adjustable time delays of course.

burn
 10 August 2017 02:25 PM
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AJJewsbury

Posts: 15634
Joined: 13 August 2003

Humm, yes I follow your thinking. My reading was "type S" or "time-delayed type with appropriate time delay setting" as far as I know, normal S-types aren't adjustable so can't have a setting as such - the latter option referring to things like earth leakage relays where often everything is adjustable.

- Andy.
 10 August 2017 03:17 PM
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OMS

Posts: 22314
Joined: 23 March 2004

Bad drafting again Andy

S types are not adjustable and in many cases no amount of ratio on the residual current rating helps as the fault current is often several orders of magnitude higher - and the protection doesn't necessarily follow and inverse definite minimum time on an RCD anyway

Table 3A and reference to BS EN 61008 and BS 61009 should show you that 100, 300 and 500mA units all have the same restraint at 1 x, 2 x and 5 x, IDelta N

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.

Edited: 10 August 2017 at 03:25 PM by OMS
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