How Fast Should A Fuse Trip If Cannot Let Go?

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

adammid

Member
Joined
Jun 26, 2012
Messages
238
Reaction score
0
Does anyone know how fast a fuse or circuit breaker must disconnect if someone is receiving a shock and cannot let go?

At approx 13-15mA it causes involuntary muscle contraction and may prevent someone from letting go.

 
Sidewinder, thats what i thought originally but think thats how fast it should trip when a 30mA current would flow. 30mA is the point at which the body can withstand without such serious consequences such as loss of consciousness hence why the regs state a 3omA rcd should trip in 0.4 seconds.

From what i have worked out i think at 13-15mA which is the point of involuntary muscle contraction a fuse should trip within 1 second but wondered if anyone else had any other thoughts.

 
Check your calcs against PD's etc and the physiological effects of current flow as published.

These documents are available from BSI, that way you can read them & understand the requirements of ADS.

Thus you can then formulate your own competent decision, if you feel my suggestion is not suitable

 
Sidewinder, thats what i thought originally but think thats how fast it should trip when a 30mA current would flow. 30mA is the point at which the body can withstand without such serious consequences such as loss of consciousness hence why the regs state a 3omA rcd should trip in 0.4 seconds.

From what i have worked out i think at 13-15mA which is the point of involuntary muscle contraction a fuse should trip within 1 second but wondered if anyone else had any other thoughts.
a fuse / MCB will never trip if you come into contact with a live wire. there is just too much resistance to gte enough current to flow

 
There is no hard and fast rule that could be used to determine the

resistance of the human body.  This means that it is almost if not

completely impossible to determine the current that would flow when

a human body is subjected to a given shock voltage, either under

conditions of direct or indirect contact.

I think Andy has hit it on the head;  the body resistance IS too

high to permit such a current to flow that would cause a protective

device to operate.

30mA was chosen as the setting for additional protection because

it is less than the 50 mA threshold at which the more serious effects

of electric shock are experienced.

I recently had a check-up for Carpal Tunnel and this involved the

passage of electric currents up each arm.  The display was clear.

Test currents were passed up each arm at 30mA.  I felt no serious

or lasting ill effects but the path of the current was nowhere near

the old ticker.

I would be interested in what someone like Apache says about this

topic.  He may confirm that livestock is FAR more susceptible to the

effects of electric shock than us humes.

Could Sidewinder post a link to that site if it is available?

 
Last edited by a moderator:
There is no hard and fast rule that could be used to determine the

resistance of the human body.  This means that it is almost if not

completely impossible to determine the current that would flow when

a human body is subjected to a given shock voltage, either under

conditions of direct or indirect contact.

I think Andy has hit it on the head;  the body resistance IS too

high to permit such a current to flow that would cause a protective

device to operate.

30mA was chosen as the setting for additional protection because

it is less than the 50 mA threshold at which the more serious effects

of electric shock are experienced.

I recently had a check-up for Carpal Tunnel and this involved the

passage of electric currents up each arm.  The display was clear.

Test currents were passed up each arm at 30mA.  I felt no serious

or lasting ill effects but the path of the current was nowhere near

the old ticker.

I would be interested in what someone like Apache says about this

topic.  He may confirm that livestock is FAR more susceptible to the

effects of electric shock than us humes.

Could Sidewinder post a link to that site if it is available?
animals are more susceptible to shocks than humans, especially from electric in the ground (lightning, dodgy cables (such as the horses at a racetrack a few years ago)), due to the increased distance between front and back feed

 
Does anyone know how fast a fuse or circuit breaker must disconnect if someone is receiving a shock and cannot let go?

At approx 13-15mA it causes involuntary muscle contraction and may prevent someone from letting go.

a fuse / MCB will never trip if you come into contact with a live wire. there is just too much resistance to gte enough current to flow

As Andy said  fuses & circuit breakers provide overload protection, the typical smallest valued used on the average installation is 6A...

So by the time any human body has 6A or more flowing through its vital organs then its pretty much too late...

However to reduce the chance of someone touching a metal part that has accidentally become live then the Fuses & Circuit breakers must disconnect in accordance with the times set out in table 41.1, regs 411.3.2,  pg 53 when a live part comes into contact with an exposed conductive part... 

The items that detect shock current are RCD's ..

Have a read of 415, where guidance on the recommended disconnection times are given.

or

section 11 of OSG pages 105 -107 where disconnection times for the various types of RCD are given..

 
Canoeboy said:
And the fact most have 4 feet not 2 and have wet feet generally and are stood in slurry, water etc etc in some circumstances

Years (thats lots) ago we used to do milking parlours, cows could feel a tingle when a 10mA trip stayed in if the conditions were right
How would you know that Canoeboy?

Was it because they started to produce milkshake :slap

 
For circuit breakers to disconnect in 0.4 sec (requirements of a TN system) a `B` type needs 3-5 times it rating to flow, a `C` type needs 10x and a `D` type 20x. So as other members have correctly said, by the time this amount is flowing you`ve had it!! 50-80 mA is the figure quote as the current that can cause fibrilation, hence an RCD that operates on the balance of current flow between the Line and Neutral conductors will sence a fault to earth (in this case through a body) and an RCD for additional protection is rated at 30mA. This should operate with an earth fault, not a circuit breaker or fuse. When testing RCD`s (of the BSEN variety) they should operate at less than 300msec for 1x their rating and less than 40msec at 5 times their rating

 
 When testing RCD`s (of the BSEN variety) they should operate at less than 300msec for 1x their rating and less than 40msec at 5 times their rating
Sorry?

The "BSEN" variety?

As compared to what?

All the RCDs I`ve ever come into contact with have a numeric descriptor for their standard compliance,

There are the older BSxxxx RCDs; but there are also many, many different "BSENxxxx" types, too.

There is NOT a generic trip time for a "BSEN"

fyi - 60898 is a "BSEN" number - but won`t trip within 300mS until you`re long dead, if you`re the shock path.

Clarification please

 
Sorry?

The "BSEN" variety?

As compared to what?

All the RCDs I`ve ever come into contact with have a numeric descriptor for their standard compliance,

There are the older BSxxxx RCDs; but there are also many, many different "BSENxxxx" types, too.

There is NOT a generic trip time for a "BSEN"

fyi - 60898 is a "BSEN" number - but won`t trip within 300mS until you`re long dead, if you`re the shock path.

Clarification please
Bs4293 are 200ms at 1x , bsen61008 are 300ms at 1x

 
Thank you Wozz for clearing that up before I got back to reply. If anyone wants to check the OSG they will see BS 4293 listed whereas all new RCD`s are BSEN 61008 or RCBO`s BSEN 61009. For some reason (beyond me) the older RCD`s must operated 100msec quicker, although how often do they ever get antwhere near that operating time in reality.

Secondly KME, if you read my post, it does state RCD`s of the BSEN variety and you go on to comment on 60898 which you`ll find are circuit breakers! Do I need to explain the difference!?

 
Last edited by a moderator:
. For some reason (beyond me) the older RCD`s must operated 100msec quicker, although how often do they ever get antwhere near that operating time in reality.

As you say 99.9999999999% of the time, the actual real world test trip times are never anywhere near 3 digit values, let alone 200ms or 300ms!

Guinness

 
BS4293 was the UK standard @ 200 ms @  1 x, but was replaced by BSEN61008 @ 300ms, to align with Europe.

Most RCDs / RCBOs trip around 20-30 ms @ 1 x and. 5 x , based on the principle of SAFETY versus AVAILABILITY. 20-30ms is the right balance between safety (trip speed) and availability without limiting availability by nuisance trip.

With regard to Agricultural applications, my understanding is that livestock  is less susceptible to shock than humans, hence no requirement for Shock protection in he Regs (705). For ADS, sockets < 32A need RCD trip @ 30mA and 100mA for > 32A. All other circuits need only be 300 mA. For Fire protection, the requirement is 300mA, with or without time delay, depending on need for continuity of service.

My own RCBOs tend to trip around 20ms for 1 x and 5 x.

 
Last edited by a moderator:

Latest posts

Top