So WHY do we test RCD's at 5x?

[springcrocus]

Following on from a post in an earlier thread (this one), an RCD is tested at half tripping current (to ensure it doesn't trip), at normal trip current (To ensure it trips within the specified time 200mS or 300mS, depending on type and ignoring type S devices) and at 5x to ensure the device trips within 40mS.

So, as in the title, WHY do we test at 5x? What is the significance of the 40mS? What are the implications of a device that doesn't open in the specified time?

Probably a daft question but I wouldn't mind getting my head round the reason.

 

[a1]

Anyone?

 

[The Godfather]

415.1.1 states that where an RCD having an IΔn of

30 mA or less is installed to provide additional

protection, its operating time should not exceed

40 ms at a residual current of 5 IΔn.

I suppose one also has to consider the implications of such current passing through the body, at 100-300mA we are on the verge of death, I'm sure we'd like to make that shock as short as possible.

The Godfather

 

[The Godfather]

RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5

 

[paulh]

If you jumped into a bath with an electric fire you would want to cut the supply asap. This test makes sure this will happen. Bit far fetched but there you go. ; \

 

[springcrocus]

Thank you for your responses, Don Vito.

I have an extract from a H&S publication (no reference, I'm afraid) that shows that at 20mA - 30mA, the body would suffer impaired breathing and that at 50mA and above, vetricular fibrillation followed by death would occur. This indicates that 50mA is a SIGNIFICANT value.

It has probably been shown that the body can withstand a current of 30mA passing through it for 300mS with no lasting effects.

However, if 50mA has been found to be the real danger level, I still ask WHY test at 5x IΔn (which is 150mA and three times the danger level) and what would be the significance of the 40mS time chosen? Are they saying that 150mA is OK for 40mS but not for, say, 60mS?

I use the 60mS figure as comparison because 5 x 60 = 300, so five times the current for a fifth of the time is, possibly, the same risk.

It all appears a little ambiguous to me. ?:|

 

[springcrocus]

If you jumped into a bath with an electric fire you would want to cut the supply asap. This test makes sure this will happen. Bit far fetched but there you go. ; \

Hi, Paul. Yes, I think we can take that as read.

I was hoping for a SLIGHTLY more technical discussion, however.

 

[kme]

The figure I was always given as the current, at which the body can have problems surviving, was 80mA.

The other major contributing factor is the route the shock takes through your body.

As an ex service engineer, it was always a good idea to have one hand in your pocket when carrying out tests on a live tv set!.

The reason? A shock between your hands is the most dangerous to your health, as the current will travel straight across the heart!

You can susvive a higher level of current, if it does not pass directly in front of, or through, the heart or brain.

I couldn`t comment on the 40mS, other than the reg. as given by ext. above..

KME

 

[a1]

KME, Please change your aviatar back - I can't get used to your current one.

 

[Apache]

Oh it's KME! Didn't spot it!

 

[a1]

He is the "Scoobmeister". :|

 

[The Godfather]

Thank you for your responses, Don Vito....It all appears a little ambiguous to me. ?:|

Thanks springcrocus, I did dig deep on this (perhaps not far enough?).

It is reasonable to assume that as the current goes beyond 30mA, the disconnection time does increase and as mentioned must be within 40mS at 150mA.

The all important issue here is time vs current.

From the information I have read, disconnection is the key issue (because of what may happen to the unlucky reciporient), so with increasing current (above 30mA) we get faster disconnection (which we would expect).

You and others have mentioned the effects at various currents, but regardless of this, if the disconnection from the supply is that brief (300mS or less, depending on the current flow), it's not likely too have any lasting effect, other than somebody in bad health.

The Godfather

 

[The Godfather]

For those that have the facility, try testing at x2 current (30mA test) and observe the difference in disconnection from x1.

Don

 

[kme]

KME, Please change your aviatar back - I get excited by your current one. Blushing

:SSorry, BOSS. Didn`t mean to upset you, BOSS!

I`ll put him to bed now, so I can get back onto the forum...............

Awwwwwww, BLESS:^O:^O:^O:^O:^O

The "Scoobster" will be back nooooooooooooooooooooooooooooooooooooow :x

 

[The Godfather]

I quite like that avatar, KME.

Don

 

[ABC Technical]

The figure I was always given as the current, at which the body can have problems surviving, was 80mA. The other major contributing factor is the route the shock takes through your body.

As an ex service engineer, it was always a good idea to have one hand in your pocket when carrying out tests on a live tv set!.

The reason? A shock between your hands is the most dangerous to your health, as the current will travel straight across the heart!

You can susvive a higher level of current, if it does not pass directly in front of, or through, the heart or brain.

I couldn`t comment on the 40mS, other than the reg. as given by ext. above..

KME

This is what I was taught too, it's a small amount but by heck it is bloody deadly.

 

[springcrocus]

As an ex service engineer, it was always a good idea to have one hand in your pocket when carrying out tests on a live tv set!.

KME

I, too , was servicing TV's (back in the late 70's) and one hand in the pocket was the norm. Most tv chassis were wired direct to neutral (or live if the plug was on wrong) so you never touched the metalwork until you had checked it! Thank heavens for the isolating transformer.

A lot of problems were with the line timebase and often the EHT voltage derived from the line flyback, so checking the 25KV to the final anode or the 8KV to the focus anode was a regular test. You soon learnt to be VERY careful and had a great respect for electricity!! I still have my old EHT meter but I think I would be too frightened to use it nowadays.

RCD's would have been very useful back then for a live chassis, but a clout from the line timebase probably wouldn't have tripped it because of the extremely low current involved. You would more likely have died from breaking your neck when you hit the wall on the other side of the room.

Sorry, drifted off topic for a while there, just reminiscing. I've nothing useful to add to the main question about why we test RCD's the way we do.

 

[Evans Electric]

I've given up tring to find the official answer to this one. I thought it was to ensure that , with direct contact with a person , a greater current flows ( unlike say 31mA leaking to earth via a damp joint box ) and the device must trip within 0.4 s at the greater current. If it doesn't its U/S . But don't quote me on this one , I'm sure the regs crunchers will be pitching in shortly. Might be tin helmet time again.

 

[kme]

I quite like that avatar, KME.Don

Which one, Mr. Don, Sir?

The "Scoobmeister", or "Oddbin"?

 

[kme]

I, too , was servicing TV's (back in the late 70's) and one hand in the pocket was the norm. Most tv chassis were wired direct to neutral (or live if the plug was on wrong) so you never touched the metalwork until you had checked it! Thank heavens for the isolating transformer.We used to do quite a few B+O sets, which had "floating chassis", or the old phillips G11 ( the one with the budgie swing!). Nasty buggr

A lot of problems were with the line timebase and often the EHT voltage derived from the line flyback, so checking the 25KV to the final anode or the 8KV to the focus anode was a regular test. You soon learnt to be VERY careful and had a great respect for electricity!! I still have my old EHT meter but I think I would be too frightened to use it nowadays.

Me too - That lovely, healthy crackling sound when the tip of the probe slid under the final anode cap:x

RCD's would have been very useful back then for a live chassis, but a clout from the line timebase probably wouldn't have tripped it because of the extremely low current involved. You would more likely have died from breaking your neck when you hit the wall on the other side of the room.

Been there - done that. Faulty valve putting a couple of KV where they had no right to be :_| . Didn`t get a t-shirt.

Sorry, drifted off topic for a while there, just reminiscing. I've nothing useful to add to the main question about why we test RCD's the way we do.

:x ]

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