Greetings! A client (Electric Utility) of ours has received a request from a developer of housing sub-divisions. They have requested that the utility use underground (submersible in vaults) transformers instead of pad-mounted transformers. The utility typically uses 50 to 100 kVA transformers serving 3 to 5 houses each at 120/240 volts. The utility anticipates using larger submersibles serving more houses. The primary voltage is 12.5/7.2 kV. The 2012 NESC table 410-1 indicates low arc energy levels (4 cal/cm squared) for pad-mounted transformers. Any experience or thoughts on PPE levels for submersible transformers in vaults.
Thanks,

I've been looking at vault equipment for some time specifically because I work in a mining environment. Everything has to be resistant to intrusion by dirt, water, and the occasional truck or bull dozer that drifts off coarse, and resistant to shock from being "gently" towed by a dozer, FREQUENTLY. Cement pads are a luxury item in this environment. Vault equipment is specifically designed to withstand being submerged in mud, water, raw sewage, etc., for an indefinite period of time. So it would seem to be a good fit with the mining environment. So far nobody I've talked to has ever considered this approach except that medium voltage elbow connectors are quickly displacing lugs on transformers because they're cheaper than paying for throats, air termination

The big problem that occurs with vaults is when you enter a medium voltage vault such as in a network distribution system where there are fairly high fault current potentials present and an arc occurs. In this case there is nowhere to hide and the tests done by IEEE and others on arc flash don't really apply because they assume either open-air conditions or else standing in front of some sort of enclosure with plenty of open air around it. In a cramped vault condition these assumptions are all violated and the incident energy is much higher. However that being the case, there are things you can do. T&B, G&W, and S&C all have breakers and switchgear specifically designed for vault mounting where the equipment is sealed and waterproofed and is operated from ground level typically with a hot stick so that the lineman does not need to enter the vault at all except in a de-energized state, especially if you spend the extra money on metering. Where you will need this will be in the 12.5 kV distribution equipment.

1. You are containing the arcing fault by placing it in a vault. This means that you will get a directed energy which can amplify things well beyond what NESC 410-1 indicates. The tabular data you are referring to generally assumes outdoor/open air conditions so the arc energy can dissipate radially in all directions which gives a substantial energy reduction. In a vault scenario, that may not exist especially if the intention is to actually enter the vault to work on the equipment. Only way to tell this is to verify it by testing at a lab (Kinetrics) but obviously this costs money. There are now "arc flash blankets" on the market that some utilities are using precisely for this scenario. There are no standards to reference so again, you'd have to test it or get manufacturer test data to rely on. Unless you are well away from the opening of the vault, while standing outside it would be best to assume IEEE 1584 test conditions in "arc-in-a-box" scenarios which provides a directed arc.
2. IEEE 1584 suggests that for 208 V or lower 3 phase circuits with a 125 kVA or smaller transformer on the secondary side, the incident energy is less than 1.2 cal/cm^2. This limit is likely to be lowered in the next edition of the standard but no way to tell what that limit will be. If you are going to stay with 50-100 kVA, you are probably still fine but I strongly recommend you read IEEE 1584 first before deciding whether or not this exception is applicable to you. If you plan on moving up to larger transformers then you'd better be doing the calculations. If you do the calculations, you will find as I have that even then if you use the "2 second rule" and assume no overcurrent protective devices, even larger transformers are frequently still below a threshold of 1.2 cal/cm^2. Again, this is either outdoor or "arc-in-a-box" where the assumption is that the worker is outside the box, not in it.
3. On the primary side of things, if you are planning on using waterproof seperable elbow connectors (IEEE 386 connectors) which are shielded for the 12.5/7.2 kV side of things, and possibly load break design at that, from a risk point of view given that the entire connector is sealed and shielded, I can't see any practical way to achieve a line-to-line arcing fault on the connector. I've heard of the connectors themselves exploding due to PD under low temperature conditions on padmount transformers where temperatures get cold enough (vault equipment tends to stay at ground temperature) over 10 years ago but most manufacturers have taken steps to eliminate this issue. I believe Cooper published quite a bit of information on the exploding connector issue. This is the design that I've recently adopted and this is also the approach (that line-to-line arcing faults are highly unlikely). Since all of my medium voltage distribution systems are also resistance grounded (even at 23 kV), the incident energy is inconsequential for line-to-ground faults.
5. You might be able to use load break elbows and might even be able to position them such that you can pull them while standing outside the vault in which case for the most part, the NESC data would apply. The rule would then be to not work on the 120/240 V stuff without de-energizing first, at least within the transformer vault. Once you are past the first protective device then even then, incident energy is likely to be extremely low. If the fuses/breakers are integrated into the transformer housing (they frequently are), then in this case you might go through some arc flash calculations assuming worst case scenarios and find that you can't generate enough energy in an arcing fault to hurt someone, unless you intend on opening the protective device compartment.

A word of warning about the IEEE 386 elbow connectors: watch out for the assumed pull ratings. The non-load break versions can be very stiff to attach or remove, but most linemen are in excellent physical shape and have only a little trouble detaching them. But the load break ones can require several hundred pounds of force and still meet specification. If you really have to use these, always make sure to leave plenty of access around the pull so that you can get on it with a couple men or perhaps even a comealong or heavy equipment in order to pull the connector off because after a few months/years of service once the lubricant has dried up, it can be really hard to detach one. The pull ratings are in the IEEE spec and well worth reading about before you go down this road.

If I were the utility, I would tell the developer there are two (2) options:

1. Take the standard above ground pad mount transformers, and provide appropriate vegetation screening while leaving plenty of access for the utility line worker to operate and service the transformer.

2. Take service at primary voltage and the developer can build the transformer vaults.

Vaults are dangerous places due to confined space access, limited work space and egress limitations.
When a fault occurs you are in a small cube that can fill rapidly with smoke, these are not safe places to work when energized.
If provisions are made in the design to allow for de-energizing the vaults for servicing and limiting the number of circuits / cables in a manhole, then the systems can be operated and maintained more safely.

Become familiar with the installation standards. Total Underground Transformers (TUT) installed for residential service will be single phase, usually installed in a small vault not designed for personnel entry, and will be connected on the primary side by load break elbow operable from outside the vault.

In the pacific northwest, a 100kW transformer might be installed to serve as many as 15 all electric homes. To put such a large size transformer in for 3-5 houses seems like overkill unless the homes are very large or have high demand loads like heat pumps. A larger transformer might not fit the standard small vault.

Anyone suggesting entry of such vault is not familiar with the work. There is no room for entry. The utility standard is usually 6-8' of clear space around such installations on the access side, so that two workers can pull on a switch stick, and there are also slide hammer type mechanisms designed to pull reluctant elbows.

I would expect the arc flash exposure of such installation to be low, by the time radiation from within an open vault gets to workers at normal working distance. One could seem to get an reasonable approximation by using the standard calculations for in a box, and applying the 3-4' working distance.

I agree this is hot stick work performed outside the enclosure just like a padmount, requiring similar PPE. Are all services brought into the vault? If so, utility will need to take an outage on all the services in order to work on just one. I'd stay with the 3-5 policy to keep things manageable. Sounds like this utility works with and stocks padmounts and not submersibles. Make sure the developer knows that he will bear the cost of the additional inventory needed to maintain this sub-division.

Your selection may have more to do with the local topography than the actual transformer load. Water Table will determine the class of equipment you use. I have had customers that used a standard pole-type transformer with elbows on the top cover instead of air insulated bushings. That standard paint job lasted about 10 years in normally dry conditions. They switched to stainless steel tanks and got 15-20 years per installation. What they also found was that over time the vaults would settle and sink which raised the inside water levels during wet periods. You will need a larger transformer because there is very little radiation cooling possible inside an earth enclosure. Your maximum loading is capped at around 100% instead of the 120% short-term overload conditions utility engineers have used as a cushion. Other situations that arose were more customer calls about "smoking" holes in the ground which were actually steam from hot transformers boiling water in the vault and more thermal run-away failures similar to what you find in dry-type transformers. When an underground unit fails, there is rarely anything worth salvaging. Any standing water in vaults should be pumped out. But you can't do that because of oil containment regulations unless the pumping is a manned operation-not a simple sump on a float switch. Prepare for lots of ground conductor corrosion and open grounds. Normal trouble-shooting operations include laying prone on the ground with your face and arms down inside the vault. Larger units have "switch into ground" functions interlocked with the incoming and outgoing circuits that are great for isolating around faulted equipment.