MK spd overcurrent protection

[Chris B]

Hi guys, I want to fit an MK spd, one of their 2-module type 2 H6820s and I can't find anywhere what kind of overcurrent protection to fit with it. Looking at the data sheets, they mention overcurrent protection being 50 kA - but that's not what I mean; what (if any) mcb should I fit with it? One of you super-experienced sparks must know... Thanks for any help!

 

[Brother Blue]

According to the data sheet the maximum rating of overcurrent protection is a 125A general service BS88 fuse.

 

[Chris B]

Thanks for that, Brother Blue. I looked on the data sheet on assets.cef.co.uk but didn't see that...?

 

[westward10]

Looking at consumer units online some are connected through a 32A mcb some are not.

 

[Chris B]

Thanks Westward10 - I saw some of those too; that's what made me ask in the first place. Confusing...

 

[Avo Mk8]

TLC seem pretty convinced it doesn't require a dedicated MCB (domestic application)

https://www.tlc-direct.co.uk/Technical/DataSheets/MK_Sentry/MK9820S%20Wiring_diagram.pdf

No ref/acknowledgement to MK spec sheet, but is consistent with the point made in post #2

 

[Brother Blue]

Thanks for that, Brother Blue. I looked on the data sheet on assets.cef.co.uk but didn't see that...?

It's in the grey box on the right hand side of that data sheet.
Max mains-side overcurrent protection: 125AgG

125A is the current rating and gG denotes a general service BS88 fuse.

 

[Chris B]

Thanks Avo Mk8 and Brother Blue. There still doesn't seem to be a definitive answer, though. MK have always made a great product, but they're a bit rubbish with their after sales info.

 

[Pretty Mouth]

As I understand it, SPDs can fail closed-circuit. If it does, then the upstream protective device should operate. Often this is an MCB, but some SPDs have their own internal protection (Hager SPDs apparently do), in which case you can omit the MCB. If the MK SPD doesn't have internal protection, and the first upstream device is the main fuse, then a failure will take out the main fuse, which isn't ideal. Might be worth you sending MK a quick email to clarify this.

 

[mickegh]

Ys my understanding was some have internal protection

Some are fitted with 32 a mcb and some don't need it

 

[Brother Blue]

Thanks Avo Mk8 and Brother Blue. There still doesn't seem to be a definitive answer, though. MK have always made a great product, but they're a bit rubbish with their after sales info.

The answer above is pretty definitive, an OCPD with a rating up to a maximum of a 125A BS88 fuse.

So you could use a fuse, MCB or MCCB of a rating below or equal to that.

 

[mickegh]

I did wonder if it's ideal to have a separate protective device upstream on a domestic SPD

There is theoretically some risk of it being cut out

 

[Chris B]

Thanks to all - there's a good dialogue going on here! Much appreciated! Pretty Mouth, I did try emailing MK - no reply. They seem to have been bought up by Novar ED&S ltd, so my query dead-ended. mickegh, it isn't obvious from the casing or from the data sheet if there is an internal mcb of some sort; Brother Blue pointed out that there is a reference to a requirement for external protection - but I have to say it is not easy to spot. Compnies like Surge Protection Devices ltd are quite willing to tell you what is required, but MK have not. Bother Blue, I do take your point that the maximum should be 125A, so, again, not definitive. Also, a company making a relatively modern device such as an spd shouldn't be going back in time by recommending a BS-88 fuse for overcurrent protection. So, yes, and mcb or an mccb would be more appropriate. But what rating? No-one seems to know for sure.

 

[Avo Mk8]

ChrisB - you might be interested in this if you have a few spare minutes!
Particularly as so far we haven't found evidence that the MK H6820s has an overload protection device inside.

View: https://youtu.be/94tJ-lpo6rY?si=k0M3beyHzSaUM7hg

 

[littlespark]

I’m not sure of MK, but every other make of SPD has had an mcb before it in the boards.
The new Proteus from CEF are making a big deal that their SPDs have an internal fuse, which frees up a way in the board.

It might be interesting to see if other brands offering the same kA surge protection have the same size mcb up front (from memory, 32A)

 

[mickegh]

Garo are max 125A backup. I looked at them recently

 

[mainline]

I’m not sure of MK, but every other make of SPD has had an mcb before it in the boards.
The new Proteus from CEF are making a big deal that their SPDs have an internal fuse, which frees up a way in the board.

It might be interesting to see if other brands offering the same kA surge protection have the same size mcb up front (from memory, 32A)

FuseBox SPD1MT2 are direct to busbar with no mcb i used one recently.

FuseBox SPD1MT2 Type 2 Single Module Surge Protection Device | Consumer Unit World

Shop for the FuseBox SPD1MT2 Type 2 Single Module Surge Protection Device. In Stock at Consumer Unit World

consumerunitworld.co.uk

So does

The Wylex NMT2SPD3W/1 a single module Type 2 Surge Protection device that has been designed for use in Wylex consumer units. It is easy to install, connecting directly to the busbar in the same way as an RCBO

 

[Pretty Mouth]

But what rating?

Ideally I would try to rate the MCB as high as I could, but would still allow selectivity with the main cutout fuse should the SPD fail short circuit. Alternatively, I'd have thought a 32A MCB would be fine, which seems to be the standard for SPDs in CU's.

Out of interest, what's the earthing arrangement for this installation?

 

[Brother Blue]

Brother Blue pointed out that there is a reference to a requirement for external protection - but I have to say it is not easy to spot.

I'd say it is as easy to spot as all of the other characteristics on that data sheet.

Bother Blue, I do take your point that the maximum should be 125A, so, again, not definitive.

How is that not definitive? It is a definite maximum rating, you work within that and establish what is appropriate for your installation. Pretty much every installation is going to be different in it's characteristics and requirements.


Are you having the same difficulty selecting a cable size because the manufacturer only tells you the maximum capacity of the terminals?

 

[Chris B]

Thanks Avo Mk8, that was an interesting vid, it's partly a sales pitch, but does have relevant insights. Thanks also to littlespark, mickegh and mainline for your suggestions of alternative brands to consider. Pretty Mouth, the earthing arrangement is TT and your thinking that 32A should be about right corresponds with the various retailers I've asked about this; they've suggested the same but without any certainty. On balance I might try for the Fusebox single module or the Wylex one. Thanks to all.

 

[Pretty Mouth]

Thanks Avo Mk8, that was an interesting vid, it's partly a sales pitch, but does have relevant insights. Thanks also to littlespark, mickegh and mainline for your suggestions of alternative brands to consider. Pretty Mouth, the earthing arrangement is TT and your thinking that 32A should be about right corresponds with the various retailers I've asked about this; they've suggested the same but without any certainty. On balance I might try for the Fusebox single module or the Wylex one. Thanks to all.

Ah, a TT earthing arrangement, there is something else we also need to consider here.

Should the SPD failed closed circuit between L and E, the low fault current that would flow in a TT system isn't going to trip the MCB. The earthing system would go live indefinitely.

To the best of my knowledge, there are two ways to overcome this:

1. Fit the SPD downstream of an RCD (as well as the MCB). Not ideal though, as transients can cause it to trip, though a time delayed RCD shouldn't suffer this AFAIK.

2. Just use the MCB, but use an SPD that uses connection type 2 (note 'connection type' is a different thing to 'type of SPD'). CT2 is arranged so you have line-SPD-neutral-SPD-earth. If the SPD fails closed circuit between L and E, then L shorts to N, tripping the MCB.

 

[sheppertonspark]

If you buy an MK populated Consumer Unit the SPD is connected directly to the Main Switch. Does this not indicate that this is how the Manufacturer intends it to be used, i.e. no MCB.

 

[mickegh]

If you buy an MK populated Consumer Unit the SPD is connected directly to the Main Switch. Does this not indicate that this is how the Manufacturer intends it to be used, i.e. no MCB.

Yes for that SPD .. Is it the same model as above query

I just Googled 6820s MK and the first link is TLC with the data sheet there on the page