PAD MOUNTED TRANSFORMER OVERCURRENT PROTECTION

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RLMJR

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British Indian Ocean Territory (Diego Garcia)
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Electrical Engineer
I am currently handling a project (replacement of pad mounted transformer). I um unsure if i need to put overcurrent protection (ECB) to protect the secondary conductors. The existing system goes like this : existing 500 kva , 13.8kv to 480/277V pad mounted transformer located outdoor with secondary conductor two sets of 500 MCM THHN (without over current protection),- connected to normal side of existing 600 amps ATS (outdoor) (open transition - not a service type ATS) (note: emergency side is connected from 300 kw generator via 600 amps breaker (outdoor) with 2 sets of 500 MCM also) : the load side of the 600 amps ATS is the connected /supplies the existing switchgear MS- MAIN 800 amps / 1200 amps rated bus.).

All are existing condition and my task is to replace the leaking defective transformer with same rating 500KVA pad mounted transformer, but i was told by my supervisor to include in the scope to put a 600 amps breaker on the secondary side of the transformer to protect the existing ATS the conductors and the loads. My question is, is this really required ? as i read in article 240.21 c (4) overcurrent protection in secondary side of transformer is not required for outside secondary conductors.


(PS; i want to share a diagram in pdf file for clear explanation but i dont know how to upload a pdf or jpeg in this post) sorry just a newbie here.

Thanks in advance for the answers.
 
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zbang

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Roughly 5346 miles from Earls Court
Required? Depends....
What rules* is this operating under? If it's considered part of the serving utility and owned by them, the secondary conductors often aren't protected at all. OTOH if it's customer/user owned, the rules are different but also depend a but on the length of the conductors- 240.21(C)(4) might apply, as could (6).

Does the ATS also have overcurrent protection? That would cover the loads, I'd expect.

*US NEC, US NESC, British rules,or some military rules
 
augie47

augie47

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Note provision (2) under 240.21(C)(4).. dos your ATS have OCP ?
 
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texie

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Location
Fort Collins, Colorado
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Electrician, Contractor, Inspector
RLMJR said:
Hi augie47, Thanks for the reply....

The ATS doesn't have an OCP or disconnect from the line side normal side, it is directly connected to the secondary side busbars of the pad mounted transformer. The ATS doesn't have an internal OCP as well.
Click to expand...
An ATS must have overcurrent protection in accordance with it's listing. The manufacture will have detailed requirements for the type, size and brand which will vary with the install conditions.
 
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RLMJR

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Location
British Indian Ocean Territory (Diego Garcia)
Occupation
Electrical Engineer
zbang said:
Required? Depends....
What rules* is this operating under? If it's considered part of the serving utility and owned by them, the secondary conductors often aren't protected at all. OTOH if it's customer/user owned, the rules are different but also depend a but on the length of the conductors- 240.21(C)(4) might apply, as could (6).

Does the ATS also have overcurrent protection? That would cover the loads, I'd expect.

*US NEC, US NESC, British rules,or some military rules
Click to expand...
Hi zbang, Thank you for the reply.

1.) The whole system including the pad mounted transformer is customer owned.

2.) The ATS doesn't have an OCP from the line side normal side. doesn't have an internal OCP as well.
 
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zbang

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Roughly 5346 miles from Earls Court
Article 110 "Requirements for Electrical Installations"
110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
Click to expand...
which includes the instruction book. (So if something isn't Listed, I guess that doesn't apply, but pretty much everything is/must be.)
 
augie47

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Here's an example from a Kohler T/S install manual:
"Note: A protective device such as a molded-case circuit breaker or fused disconnect switch MUST be
installed on both sources of incoming power for circuit protection and used as a disconnect device"
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
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paulengr

Senior Member
You protect a transformer on the primary side from short circuit only. Primary side only protection can work in very small transformers only, not this large. The utility will follow NESC. They can but don’t have to provide secondary side protection. Often the secondary side and the transformer itself is protection by a secondary side OCPD by the tap rules and the assumption that damage is unlikely.

There are a couple ways to do it. The most obvious is get a 600 A “breaker in a box” or a 600 A fused disconnect and call it a day. But there are other options. First thing is have you looked at arc flash ratings? I’ll give you a hint: anyone operating that disconnect/breaker is taking a huge safety risk. Chances are no PPE is going to be rated high enough. So a breaker or fused disconnect is rather silly. All load breaking should be done on the 13.8 kV side using the ATS to lock out against backfeeds.

I would look at CIBA or class L fuses, no disconnect (waste of money). This gets you what you want and makes it impossible for someone to get into trouble.

Another, usually cheaper option is this. Install bushing CTs on the secondary side and a simple 15 kV vacuum breaker on the primary side with a non-load break switch. Set up two protective relays. One does the primary protection and the other does secondary but they both trip the VCB. That way you get the ultimate in protection but without the extremely high AIC of the 480 V side.
 
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